You can do that using Scott Dorman s Visual Studio macro available on CodeProject:

Visual Studio 2010 and Target Framework Version

Below is the code, download it to your <UserProfile>DocumentsVisual Studio 2010ProjectsVSMacros80MyMacros folder, open the Visual Studio Macro IDE (Alt-F11) and add it as an existing item to the “MyMacros” project:

 ------------------------------------------------------------------------------
  Visual Studio 2008 Macros
 
  ProjectUtilities.vb
 
 ------------------------------------------------------------------------------
  Copyright (C) 2007-2008 Scott Dorman (sj_dorman@hotmail.com)
 
  This library is free software; you can redistribute it and/or
  modify it under the terms of the Microsoft Public License (Ms-PL).
 
  This library is distributed in the hope that it will be useful,
  but WITHOUT ANY WARRANTY; without even the implied warranty of
  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
  Microsoft Public License (Ms-PL) for more details.
 ------------------------------------------------------------------------------
Imports System
Imports EnvDTE
Imports EnvDTE80
Imports EnvDTE90
Imports System.Diagnostics

Public Module ProjectUtilities

    Private Class ProjectGuids
        Public Const vsWindowsCSharp As String = "{FAE04EC0-301F-11D3-BF4B-00C04F79EFBC}"
        Public Const vsWindowsVBNET As String = "{F184B08F-C81C-45F6-A57F-5ABD9991F28F}"
        Public Const vsWindowsVisualCPP As String = "{8BC9CEB8-8B4A-11D0-8D11-00A0C91BC942}"
        Public Const vsWebApplication As String = "{349C5851-65DF-11DA-9384-00065B846F21}"
        Public Const vsWebSite As String = "{E24C65DC-7377-472B-9ABA-BC803B73C61A}"
        Public Const vsDistributedSystem As String = "{F135691A-BF7E-435D-8960-F99683D2D49C}"
        Public Const vsWCF As String = "{3D9AD99F-2412-4246-B90B-4EAA41C64699}"
        Public Const vsWPF As String = "{60DC8134-EBA5-43B8-BCC9-BB4BC16C2548}"
        Public Const vsVisualDatabaseTools As String = "{C252FEB5-A946-4202-B1D4-9916A0590387}"
        Public Const vsDatabase As String = "{A9ACE9BB-CECE-4E62-9AA4-C7E7C5BD2124}"
        Public Const vsDatabaseOther As String = "{4F174C21-8C12-11D0-8340-0000F80270F8}"
        Public Const vsTest As String = "{3AC096D0-A1C2-E12C-1390-A8335801FDAB}"
        Public Const vsLegacy2003SmartDeviceCSharp As String = "{20D4826A-C6FA-45DB-90F4-C717570B9F32}"
        Public Const vsLegacy2003SmartDeviceVBNET As String = "{CB4CE8C6-1BDB-4DC7-A4D3-65A1999772F8}"
        Public Const vsSmartDeviceCSharp As String = "{4D628B5B-2FBC-4AA6-8C16-197242AEB884}"
        Public Const vsSmartDeviceVBNET As String = "{68B1623D-7FB9-47D8-8664-7ECEA3297D4F}"
        Public Const vsWorkflowCSharp As String = "{14822709-B5A1-4724-98CA-57A101D1B079}"
        Public Const vsWorkflowVBNET As String = "{D59BE175-2ED0-4C54-BE3D-CDAA9F3214C8}"
        Public Const vsDeploymentMergeModule As String = "{06A35CCD-C46D-44D5-987B-CF40FF872267}"
        Public Const vsDeploymentCab As String = "{3EA9E505-35AC-4774-B492-AD1749C4943A}"
        Public Const vsDeploymentSetup As String = "{978C614F-708E-4E1A-B201-565925725DBA}"
        Public Const vsDeploymentSmartDeviceCab As String = "{AB322303-2255-48EF-A496-5904EB18DA55}"
        Public Const vsVSTA As String = "{A860303F-1F3F-4691-B57E-529FC101A107}"
        Public Const vsVSTO As String = "{BAA0C2D2-18E2-41B9-852F-F413020CAA33}"
        Public Const vsSharePointWorkflow As String = "{F8810EC1-6754-47FC-A15F-DFABD2E3FA90}"
    End Class

       Defines the valid target framework values.
    Enum TargetFramework
        Fx40 = 262144
        Fx35 = 196613
        Fx30 = 196608
        Fx20 = 131072
    End Enum

       Change the target framework for all projects in the current solution.
    Sub ChangeTargetFrameworkForAllProjects()
        Dim project As EnvDTE.Project
        Dim clientProfile As Boolean = False

        Write("--------- CHANGING TARGET .NET FRAMEWORK VERSION -------------")
        Try
            If Not DTE.Solution.IsOpen Then
                Write("There is no solution open.")
            Else              
                Dim targetFrameworkInput As String = InputBox("Enter the target framework version (Fx40, Fx35, Fx30, Fx20):", "Target Framework", "Fx40")
                Dim targetFramework As TargetFramework = [Enum].Parse(GetType(TargetFramework), targetFrameworkInput)

                If targetFramework = ProjectUtilities.TargetFramework.Fx35 Or targetFramework = ProjectUtilities.TargetFramework.Fx40 Then
                    Dim result As MsgBoxResult = MsgBox("The .NET Framework version chosen supports a Client Profile. Would you like to use that profile?", MsgBoxStyle.Question Or MsgBoxStyle.YesNo, "Target Framework Profile")
                    If result = MsgBoxResult.Yes Then
                        clientProfile = True
                    End If
                End If

                For Each project In DTE.Solution.Projects
                    If project.Kind <> Constants.vsProjectKindSolutionItems And project.Kind <> Constants.vsProjectKindMisc Then
                        ChangeTargetFramework(project, targetFramework, clientProfile)
                    Else
                        For Each projectItem In project.ProjectItems
                            If Not (projectItem.SubProject Is Nothing) Then
                                ChangeTargetFramework(projectItem.SubProject, targetFramework, clientProfile)
                            End If
                        Next

                    End If
                Next
            End If
        Catch ex As System.Exception
            Write(ex.Message)
        End Try
    End Sub

       Change the target framework for a project.
    Function ChangeTargetFramework(ByVal project As EnvDTE.Project, ByVal targetFramework As TargetFramework, ByVal clientProfile As Boolean) As Boolean
        Dim changed As Boolean = True

        If project.Kind = Constants.vsProjectKindSolutionItems Or project.Kind = Constants.vsProjectKindMisc Then
            For Each projectItem In project.ProjectItems
                If Not (projectItem.SubProject Is Nothing) Then
                    ChangeTargetFramework(projectItem.SubProject, targetFramework, clientProfile)
                End If
            Next
        Else
            Try
                If IsLegalProjectType(project) Then
                    SetTargetFramework(project, targetFramework, clientProfile)
                Else
                    Write("Skipping project: " + project.Name + " (" + project.Kind + ")")
                End If
            Catch ex As Exception
                Write(ex.Message)
                changed = False
            End Try
        End If

        Return changed
    End Function

       Determines if the project is a project that actually supports changing the target framework.
    Function IsLegalProjectType(ByVal proejct As EnvDTE.Project) As Boolean
        Dim legalProjectType As Boolean = True

        Select Case proejct.Kind
            Case ProjectGuids.vsDatabase
                legalProjectType = False
            Case ProjectGuids.vsDatabaseOther
                legalProjectType = False
            Case ProjectGuids.vsDeploymentCab
                legalProjectType = False
            Case ProjectGuids.vsDeploymentMergeModule
                legalProjectType = False
            Case ProjectGuids.vsDeploymentSetup
                legalProjectType = False
            Case ProjectGuids.vsDeploymentSmartDeviceCab
                legalProjectType = False
            Case ProjectGuids.vsDistributedSystem
                legalProjectType = False
            Case ProjectGuids.vsLegacy2003SmartDeviceCSharp
                legalProjectType = False
            Case ProjectGuids.vsLegacy2003SmartDeviceVBNET
                legalProjectType = False
            Case ProjectGuids.vsSharePointWorkflow
                legalProjectType = False
            Case ProjectGuids.vsSmartDeviceCSharp
                legalProjectType = True
            Case ProjectGuids.vsSmartDeviceVBNET
                legalProjectType = True
            Case ProjectGuids.vsTest
                legalProjectType = False
            Case ProjectGuids.vsVisualDatabaseTools
                legalProjectType = False
            Case ProjectGuids.vsVSTA
                legalProjectType = True
            Case ProjectGuids.vsVSTO
                legalProjectType = True
            Case ProjectGuids.vsWCF
                legalProjectType = True
            Case ProjectGuids.vsWebApplication
                legalProjectType = True
            Case ProjectGuids.vsWebSite
                legalProjectType = True
            Case ProjectGuids.vsWindowsCSharp
                legalProjectType = True
            Case ProjectGuids.vsWindowsVBNET
                legalProjectType = True
            Case ProjectGuids.vsWindowsVisualCPP
                legalProjectType = True
            Case ProjectGuids.vsWorkflowCSharp
                legalProjectType = False
            Case ProjectGuids.vsWorkflowVBNET
                legalProjectType = False
            Case ProjectGuids.vsWPF
                legalProjectType = True
            Case Else
                legalProjectType = False
        End Select
        Return legalProjectType
    End Function

       Sets the target framework for the project to the specified framework.
    Sub SetTargetFramework(ByVal project As EnvDTE.Project, ByVal targetFramework As TargetFramework, ByVal clientProfile As Boolean)
        Dim currentTargetFramework As TargetFramework = CType(project.Properties.Item("TargetFramework").Value, TargetFramework)
        Dim targetMoniker As String = GetTargetFrameworkMoniker(targetFramework, clientProfile)
        Dim currentMoniker As String = project.Properties.Item("TargetFrameworkMoniker").Value

        If currentMoniker <> targetMoniker Then
            Write("Changing project: " + project.Name + " from " + currentMoniker + " to " + targetMoniker + ".")
            project.Properties.Item("TargetFrameworkMoniker").Value = targetMoniker
            project.Properties.Item("TargetFramework").Value = targetFramework
        Else
            Write("Skipping project: " + project.Name + ", already at the correct target framework.")
        End If
    End Sub

    Function GetTargetFrameworkMoniker(ByVal targetFramework As TargetFramework, ByVal clientProfile As Boolean) As String
        Dim moniker As String = ".NETFramework,Version=v"
        Select Case targetFramework
            Case ProjectUtilities.TargetFramework.Fx20
                moniker += "2.0"

            Case ProjectUtilities.TargetFramework.Fx30
                moniker += "3.0"

            Case ProjectUtilities.TargetFramework.Fx35
                moniker += "3.5"

            Case ProjectUtilities.TargetFramework.Fx40
                moniker += "4.0"

        End Select

        If clientProfile Then
            moniker += ",Profile=Client"
        End If

        Return moniker
    End Function

       Writes a message to the output window
    Sub Write(ByVal s As String)
        Dim out As OutputWindowPane = GetOutputWindowPane("Change Target Framework", True)
        out.OutputString(s)
        out.OutputString(vbCrLf)
    End Sub

       Gets an instance of the output window
    Function GetOutputWindowPane(ByVal Name As String, Optional ByVal show As Boolean = True) As OutputWindowPane
        Dim win As Window = DTE.Windows.Item(EnvDTE.Constants.vsWindowKindOutput)
        If show Then win.Visible = True
        Dim ow As OutputWindow = win.Object
        Dim owpane As OutputWindowPane
        Try
            owpane = ow.OutputWindowPanes.Item(Name)
        Catch e As System.Exception
            owpane = ow.OutputWindowPanes.Add(Name)
        End Try
        owpane.Activate()
        Return owpane
    End Function

End Module

Just published Target Framework Migrator, Visual Studio Extension for changing multiple .Net projects Target Framework at once

A PowerShell script that I used to do mine. Admittedly bruce force-ish.

Get-ChildItem . -Recurse -Filter *.*proj |ForEach {
    $content = Get-Content $_.FullName
    $content |ForEach {
        $_.Replace("<TargetFrameworkVersion>v4.0</TargetFrameworkVersion>", "<TargetFrameworkVersion>v4.5</TargetFrameworkVersion>")
    } |Set-Content $_.FullName
}

There s always the simple. A decent text editor such as notepad++ will include a find/replace in files function. Just search for the current version string in your csproj/vbproj files:

<TargetFrameworkVersion>v3.5</TargetFrameworkVersion>

and replace with the new version

<TargetFrameworkVersion>v4.5</TargetFrameworkVersion>

Good idea to check out first though...

From bash:

$find . -name "*.csproj" -exec sed -b -i "s,<TargetFrameworkVersion>[^<]*</TargetFrameworkVersion>,<TargetFrameworkVersion>v4.6.1</TargetFrameworkVersion>," {} ;

I think by far the easiest way to do this is using a search and replace tool. It is an advantage if it supports regex.

There must be quite a few out there - the first one I tested worked for me, though: http://www.ecobyte.com/replacetext/

There is a note saying it has some issues on Win7, but I didn t experience that.

Step by step instructions in that tool:

1. Replace | Add Group | Name it (e.g. "MyGroup")
2. Right-Click MyGroup | Add File(s)...
3. Choose your source (e.g. Use Folder, browse to root folder of the projects you want to change)
4. Set the Include File Filter if necessary (e.g. *.csproj)
5. Right-Click the row below Original Text | Advanced Edit...
6. Enter your regular expression in Search Text Box (e.g. <TargetFrameworkVersion>.*</TargetFrameworkVersion>)
7. Select "Regular Expression Search" in the combobox below the Search Text
8. Enter Replacement Text (e.g. <TargetFrameworkVersion>4.0</TargetFrameworkVersion>)
9. Choose Destination and Backup settings (will create a backup by default)
10. Start Replacing (Ctrl+R)

Now If for some reason you need to do this in code, I would probably be able to do that as well (it s how I found this question). In that case, please request it in a comment.

Target Framework Migrator by Pavel Samokha (VHQLabs)

http://visualstudiogallery.msdn.microsoft.com/47bded90-80d8-42af-bc35-4736fdd8cd13

Conditional regexes are giving me a headache. So here is a coded solution for search/replace (I m avoiding EnvDTE as long as possible).

Order does not seem to matter for the project file entries:

Try something along those lines:

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.IO;
using System.Text.RegularExpressions;

namespace TextReplaceDemo
{
    class Program
    {
        static void Main(string[] args)
        {
            try
            {
                ReplaceTargetFrameworkVersion("v4.0", "c:/projekt/2005", "*.csproj");
            }
            catch (Exception ex)
            {
                Console.WriteLine(ex.Message);
            }
        }

        /// <summary>
        /// Inserts the denoted targetFramework into all occurrences of TargetFrameworkVersion.
        /// If the TargetFrameworkVersion is not present in the file, the method searches for the 
        /// OutputType tag, which should be present, and inserts the TargetFrameworkVersion before that.
        /// </summary>
        /// <param name="targetFramework">New target framework (e.g. "v4.0")</param>
        /// <param name="rootDirectory">Root directory for the file search (e.g. "c:Projects2005")</param>
        /// <param name="fileSearchPattern">Pattern to find the project files (e.g. "*.csproj). 
        /// Will get all files for empty parameter.</param>
        public static void ReplaceTargetFrameworkVersion(string targetFramework, string rootDirectory, string fileSearchPattern)
        {
            if (string.IsNullOrEmpty(targetFramework)) throw new ArgumentNullException("targetFramework");
            if (string.IsNullOrEmpty(rootDirectory)) throw new ArgumentNullException("rootDirectory");
            if (string.IsNullOrEmpty(fileSearchPattern)) fileSearchPattern = "*.*";

            string regexPattern = "<TargetFrameworkVersion>.*</TargetFrameworkVersion>";
            string insertText = string.Format("<TargetFrameworkVersion>{0}</TargetFrameworkVersion>", targetFramework);
            string alternativeMarker = "<OutputType>";

            // get all files
            List<FileInfo> files = GetAllFiles(rootDirectory, fileSearchPattern);

            // iterate over found files
            foreach (var file in files)
            {
                string fileContent = File.ReadAllText(file.FullName);
                Match match = Regex.Match(fileContent, regexPattern);
                string newfileContent = null;
                if (match.Success)
                {
                    // replace <TargetFrameworkVersion>
                    newfileContent = fileContent.Replace(match.Value, insertText);
                }
                else if (fileContent.Contains(alternativeMarker))
                {
                    // create <TargetFrameworkVersion>
                    newfileContent = fileContent.Replace(alternativeMarker,
                        insertText + Environment.NewLine + "    " + alternativeMarker);
                }

                // overwrite file
                if (newfileContent != null)
                    File.WriteAllText(file.FullName, newfileContent);
            }
        }


        /// <summary>
        /// Recursive function to find all files in a directory by a searchPattern
        /// </summary>
        /// <param name="path">Path to the root directory</param>
        /// <param name="searchPattern">Pattern for the file search, e.g. "*.txt"</param>
        public static List<FileInfo> GetAllFiles(string path, string searchPattern)
        {
            List<FileInfo> files = new List<FileInfo>();

            DirectoryInfo dir = new DirectoryInfo(path);

            if (dir.Exists)
            {
                // get all files in directory
                files.AddRange(dir.GetFiles(searchPattern));

                // get all files of subdirectories
                foreach (var subDir in dir.GetDirectories())
                {
                    files.AddRange(GetAllFiles(subDir.FullName, searchPattern));
                }
            }
            return files;
        }
    }
}

You can use a macro to do this, or remember that the VS project files are text files, which means a simple global search and replace can achieve the same, and is a more general technique for making the same change to lots of project files.

First back up an existing project file, then make the change you want (e.g., change the target framework). Use WinDiff or WinMerge to compare the new project file with the back up. This will tell you the change you need to make. Then use the Visual Studio IDE s Find and Replace in Files function to make the change to all your project files.

An alternative with no external tools (and the ability to change other settings, e.g. ToolsVersion):

1. Using Visual Studio, create a new c# console project ManageCSProjFiles (or whatever name you like) in a temporary directory. Be sure to check Place solution and project in same directory
2. Remove Program.cs and Properties/AssemblyInfo.cs from the project. There is no need to ever compile the project.
3. Save the project and close Visual Studio.
4. Using a text editor open ManageCSProjFiles.csproj. Add the following lines at the bottom, before the last line:

  <ItemGroup>
    <None Include="***.csproj" />
  </ItemGroup>

5. Copy ManageCSProjFiles.sln and ManageCSProjFiles.csproj to the topmost directory of your solution tree.
6. If you load the ManageCSProjFiles solution in Visual Studio, it will now show all your .csproj files and you can change them with the search/replace tools in Visual Studio.

This can easily be extended for other project types.