What are the project GUIDs in a Visual Studio solution file used for?

I have multiple projects in a single Visual Studio (2008) solution.
I just discovered that each of these projects uses a same GUID, so in the solution file it looks like this:

Project("{FAE04EC0-F103-D311-BF4B-00C04FCBFE97}") = "Pro1", "Pro1\Pro1.csproj", "{...}"
Project("{FAE04EC0-F103-D311-BF4B-00C04FCBFE97}") = "Pro2", "Pro2\Pro2.csproj", "{...}"

Do I have to change these GUIDs so they're unique and what are they used for?

Project persistence block in a solution file has the following format:

Project("{project type GUID}") = "<Project name>", "<project file location>", 
    "{<Unique project GUID>}"
EndProject

So it's expected that first GUID is non-unique, it uniquely identifies Visual Studio package that handles this type of projects.

The GUID you posted is interesting - it looks like mangled C# project GUID, which is FAE04EC0-301F-11D3-BF4B-00C04F79EFBC. Did you change it yourself or is it a posting issue (e.g. you are using right-to-left locale such as Hebrew or Arabic).

The first GUID is the project type. Project type GUIDs can be found here:

http://www.codeproject.com/Reference/720512/List-of-Visual-Studio-Project-Type-GUIDs

From the link:

There isn't an easy way to change the type of a project in Visual Studio project once it is created; for example changing a C# Class Library into a Portable Class Library (PCL). Knowing these GUIDs will allow you to edit project files like .csproj and .vbproj to easily change a project type in cases where Visual Studio will not allow using the GUI.

By description/project type:

+------------------------------------------------+----------------------------------------+
|                  Project Type                  |                 GUID        
+------------------------------------------------+----------------------------------------+
| .NET Core                                      | {9A19103F-16F7-4668-BE54-9A1E7A4F7556} |
| ASP.NET 5                                      | {8BB2217D-0F2D-49D1-97BC-3654ED321F3B} |
| ASP.NET MVC 1                                  | {603C0E0B-DB56-11DC-BE95-000D561079B0} |
| ASP.NET MVC 2                                  | {F85E285D-A4E0-4152-9332-AB1D724D3325} |
| ASP.NET MVC 3                                  | {E53F8FEA-EAE0-44A6-8774-FFD645390401} |
| ASP.NET MVC 4                                  | {E3E379DF-F4C6-4180-9B81-6769533ABE47} |
| ASP.NET MVC 5                                  | {349C5851-65DF-11DA-9384-00065B846F21} |
| C#                                             | {FAE04EC0-301F-11D3-BF4B-00C04F79EFBC} |
| C++                                            | {8BC9CEB8-8B4A-11D0-8D11-00A0C91BC942} |
| Database                                       | {A9ACE9BB-CECE-4E62-9AA4-C7E7C5BD2124} |
| Database (other project types)                 | {4F174C21-8C12-11D0-8340-0000F80270F8} |
| Deployment Cab                                 | {3EA9E505-35AC-4774-B492-AD1749C4943A} |
| Deployment Merge Module                        | {06A35CCD-C46D-44D5-987B-CF40FF872267} |
| Deployment Setup                               | {978C614F-708E-4E1A-B201-565925725DBA} |
| Deployment Smart Device Cab                    | {AB322303-2255-48EF-A496-5904EB18DA55} |
| Distributed System                             | {F135691A-BF7E-435D-8960-F99683D2D49C} |
| Dynamics 2012 AX C# in AOT                     | {BF6F8E12-879D-49E7-ADF0-5503146B24B8} |
| F#                                             | {F2A71F9B-5D33-465A-A702-920D77279786} |
| J#                                             | {E6FDF86B-F3D1-11D4-8576-0002A516ECE8} |
| Legacy (2003) Smart Device (C#)                | {20D4826A-C6FA-45DB-90F4-C717570B9F32} |
| Legacy (2003) Smart Device (VB.NET)            | {CB4CE8C6-1BDB-4DC7-A4D3-65A1999772F8} |
| Micro Framework                                | {b69e3092-b931-443c-abe7-7e7b65f2a37f} |
| Model-View-Controller v2 (MVC 2)               | {F85E285D-A4E0-4152-9332-AB1D724D3325} |
| Model-View-Controller v3 (MVC 3)               | {E53F8FEA-EAE0-44A6-8774-FFD645390401} |
| Model-View-Controller v4 (MVC 4)               | {E3E379DF-F4C6-4180-9B81-6769533ABE47} |
| Model-View-Controller v5 (MVC 5)               | {349C5851-65DF-11DA-9384-00065B846F21} |
| Mono for Android                               | {EFBA0AD7-5A72-4C68-AF49-83D382785DCF} |
| MonoTouch                                      | {6BC8ED88-2882-458C-8E55-DFD12B67127B} |
| MonoTouch Binding                              | {F5B4F3BC-B597-4E2B-B552-EF5D8A32436F} |
| Portable Class Library                         | {786C830F-07A1-408B-BD7F-6EE04809D6DB} |
| Project Folders                                | {66A26720-8FB5-11D2-AA7E-00C04F688DDE} |
| Service Fabric Application (.sfproj)           | {A07B5EB6-E848-4116-A8D0-A826331D98C6} |
| SharePoint (C#)                                | {593B0543-81F6-4436-BA1E-4747859CAAE2} |
| SharePoint (VB.NET)                            | {EC05E597-79D4-47f3-ADA0-324C4F7C7484} |
| SharePoint Workflow                            | {F8810EC1-6754-47FC-A15F-DFABD2E3FA90} |
| Silverlight                                    | {A1591282-1198-4647-A2B1-27E5FF5F6F3B} |
| Smart Device (C#)                              | {4D628B5B-2FBC-4AA6-8C16-197242AEB884} |
| Smart Device (VB.NET)                          | {68B1623D-7FB9-47D8-8664-7ECEA3297D4F} |
| Solution Folder                                | {2150E333-8FDC-42A3-9474-1A3956D46DE8} |
| Test                                           | {3AC096D0-A1C2-E12C-1390-A8335801FDAB} |
| Universal Windows Class Library                | {A5A43C5B-DE2A-4C0C-9213-0A381AF9435A} |
| VB.NET                                         | {F184B08F-C81C-45F6-A57F-5ABD9991F28F} |
| Visual Database Tools                          | {C252FEB5-A946-4202-B1D4-9916A0590387} |
| Visual Studio 2015 Installer Project Extension | {54435603-DBB4-11D2-8724-00A0C9A8B90C} |
| Visual Studio Tools for Applications (VSTA)    | {A860303F-1F3F-4691-B57E-529FC101A107} |
| Visual Studio Tools for Office (VSTO)          | {BAA0C2D2-18E2-41B9-852F-F413020CAA33} |
| Web Application                                | {349C5851-65DF-11DA-9384-00065B846F21} |
| Web Site                                       | {E24C65DC-7377-472B-9ABA-BC803B73C61A} |
| Windows (C#)                                   | {FAE04EC0-301F-11D3-BF4B-00C04F79EFBC} |
| Windows (VB.NET)                               | {F184B08F-C81C-45F6-A57F-5ABD9991F28F} |
| Windows (Visual C++)                           | {8BC9CEB8-8B4A-11D0-8D11-00A0C91BC942} |
| Windows Communication Foundation (WCF)         | {3D9AD99F-2412-4246-B90B-4EAA41C64699} |
| Windows Phone 8/8.1 Blank/Hub/Webview App      | {76F1466A-8B6D-4E39-A767-685A06062A39} |
| Windows Phone 8/8.1 App (C#)                   | {C089C8C0-30E0-4E22-80C0-CE093F111A43} |
| Windows Phone 8/8.1 App (VB.NET)               | {DB03555F-0C8B-43BE-9FF9-57896B3C5E56} |
| Windows Presentation Foundation (WPF)          | {60DC8134-EBA5-43B8-BCC9-BB4BC16C2548} |
| Windows Store (Metro) Apps & Components        | {BC8A1FFA-BEE3-4634-8014-F334798102B3} |
| Workflow (C#)                                  | {14822709-B5A1-4724-98CA-57A101D1B079} |
| Workflow (VB.NET)                              | {D59BE175-2ED0-4C54-BE3D-CDAA9F3214C8} |
| Workflow Foundation                            | {32F31D43-81CC-4C15-9DE6-3FC5453562B6} |
| Xamarin.Android                                | {EFBA0AD7-5A72-4C68-AF49-83D382785DCF} |
| Xamarin.iOS                                    | {6BC8ED88-2882-458C-8E55-DFD12B67127B} |
| XNA (Windows)                                  | {6D335F3A-9D43-41b4-9D22-F6F17C4BE596} |
| XNA (XBox)                                     | {2DF5C3F4-5A5F-47a9-8E94-23B4456F55E2} |
| XNA (Zune)                                     | {D399B71A-8929-442a-A9AC-8BEC78BB2433} |
+------------------------------------------------+----------------------------------------+

By GUID:

+------------------------------------------------+----------------------------------------+
|                  Project Type                  |                 GUID  
+------------------------------------------------+----------------------------------------+
| Deployment Merge Module                        | {06A35CCD-C46D-44D5-987B-CF40FF872267} |
| Workflow (C#)                                  | {14822709-B5A1-4724-98CA-57A101D1B079} |
| Legacy (2003) Smart Device (C#)                | {20D4826A-C6FA-45DB-90F4-C717570B9F32} |
| Solution Folder                                | {2150E333-8FDC-42A3-9474-1A3956D46DE8} |
| XNA (XBox)                                     | {2DF5C3F4-5A5F-47a9-8E94-23B4456F55E2} |
| Workflow Foundation                            | {32F31D43-81CC-4C15-9DE6-3FC5453562B6} |
| Web Application (incl. MVC 5)                  | {349C5851-65DF-11DA-9384-00065B846F21} |
| Test                                           | {3AC096D0-A1C2-E12C-1390-A8335801FDAB} |
| Windows Communication Foundation (WCF)         | {3D9AD99F-2412-4246-B90B-4EAA41C64699} |
| Deployment Cab                                 | {3EA9E505-35AC-4774-B492-AD1749C4943A} |
| Smart Device (C#)                              | {4D628B5B-2FBC-4AA6-8C16-197242AEB884} |
| Database (other project types)                 | {4F174C21-8C12-11D0-8340-0000F80270F8} |
| Visual Studio 2015 Installer Project Extension | {54435603-DBB4-11D2-8724-00A0C9A8B90C} |
| SharePoint (C#)                                | {593B0543-81F6-4436-BA1E-4747859CAAE2} |
| ASP.NET MVC 1.0                                | {603C0E0B-DB56-11DC-BE95-000D561079B0} |
| Windows Presentation Foundation (WPF)          | {60DC8134-EBA5-43B8-BCC9-BB4BC16C2548} |
| Smart Device (VB.NET)                          | {68B1623D-7FB9-47D8-8664-7ECEA3297D4F} |
| Project Folders                                | {66A26720-8FB5-11D2-AA7E-00C04F688DDE} |
| MonoTouch                                      | {6BC8ED88-2882-458C-8E55-DFD12B67127B} |
| XNA (Windows)                                  | {6D335F3A-9D43-41b4-9D22-F6F17C4BE596} |
| Windows Phone 8/8.1 Blank/Hub/Webview App      | {76F1466A-8B6D-4E39-A767-685A06062A39} |
| Portable Class Library                         | {786C830F-07A1-408B-BD7F-6EE04809D6DB} |
| ASP.NET 5                                      | {8BB2217D-0F2D-49D1-97BC-3654ED321F3B} |
| C++                                            | {8BC9CEB8-8B4A-11D0-8D11-00A0C91BC942} |
| Deployment Setup                               | {978C614F-708E-4E1A-B201-565925725DBA} |
| .NET Core                                      | {9A19103F-16F7-4668-BE54-9A1E7A4F7556} |
| Service Fabric Application (.sfproj)           | {A07B5EB6-E848-4116-A8D0-A826331D98C6} |
| Silverlight                                    | {A1591282-1198-4647-A2B1-27E5FF5F6F3B} |
| Universal Windows Class Library                | {A5A43C5B-DE2A-4C0C-9213-0A381AF9435A} |
| Visual Studio Tools for Applications (VSTA)    | {A860303F-1F3F-4691-B57E-529FC101A107} |
| Database                                       | {A9ACE9BB-CECE-4E62-9AA4-C7E7C5BD2124} |
| Deployment Smart Device Cab                    | {AB322303-2255-48EF-A496-5904EB18DA55} |
| Micro Framework                                | {B69E3092-B931-443C-ABE7-7E7B65F2A37F} |
| Visual Studio Tools for Office (VSTO)          | {BAA0C2D2-18E2-41B9-852F-F413020CAA33} |
| Windows Store Apps (Metro Apps)                | {BC8A1FFA-BEE3-4634-8014-F334798102B3} |
| C# in Dynamics 2012 AX AOT                     | {BF6F8E12-879D-49E7-ADF0-5503146B24B8} |
| Windows Phone 8/8.1 App (C#)                   | {C089C8C0-30E0-4E22-80C0-CE093F111A43} |
| Visual Database Tools                          | {C252FEB5-A946-4202-B1D4-9916A0590387} |
| Legacy (2003) Smart Device (VB.NET)            | {CB4CE8C6-1BDB-4DC7-A4D3-65A1999772F8} |
| XNA (Zune)                                     | {D399B71A-8929-442a-A9AC-8BEC78BB2433} |
| Workflow (VB.NET)                              | {D59BE175-2ED0-4C54-BE3D-CDAA9F3214C8} |
| Windows Phone 8/8.1 App (VB.NET)               | {DB03555F-0C8B-43BE-9FF9-57896B3C5E56} |
| Web Site                                       | {E24C65DC-7377-472B-9ABA-BC803B73C61A} |
| ASP.NET MVC 4.0                                | {E3E379DF-F4C6-4180-9B81-6769533ABE47} |
| ASP.NET MVC 3.0                                | {E53F8FEA-EAE0-44A6-8774-FFD645390401} |
| J#                                             | {E6FDF86B-F3D1-11D4-8576-0002A516ECE8} |
| SharePoint (VB.NET)                            | {EC05E597-79D4-47f3-ADA0-324C4F7C7484} |
| Xamarin.Android / Mono for Android             | {EFBA0AD7-5A72-4C68-AF49-83D382785DCF} |
| Distributed System                             | {F135691A-BF7E-435D-8960-F99683D2D49C} |
| VB.NET                                         | {F184B08F-C81C-45F6-A57F-5ABD9991F28F} |
| F#                                             | {F2A71F9B-5D33-465A-A702-920D77279786} |
| MonoTouch Binding                              | {F5B4F3BC-B597-4E2B-B552-EF5D8A32436F} |
| ASP.NET MVC 2.0                                | {F85E285D-A4E0-4152-9332-AB1D724D3325} |
| SharePoint Workflow                            | {F8810EC1-6754-47FC-A15F-DFABD2E3FA90} |
| C#                                             | {FAE04EC0-301F-11D3-BF4B-00C04F79EFBC} |
+------------------------------------------------+----------------------------------------+

Notes: You will find these properties as a <ProjectTypeGuids> element in the project file, not to be confused with <ProjectGuid> which is a unique identifier for your project.

Some GUIDs are used in combination. For example, <ProjectTypeGuids>{EFBA0AD7-5A72-4C68-AF49-83D382785DCF};{FAE04EC0-301F-11D3-BF4B-00C04F79EFBC}</ProjectTypeGuids> describes a Xamarin.Android library in C#.

I will often copy a VS project file, rename it and search and replace all occurrences of the project name to create a new project with same settings. This is often quicker than creating a new project and then ensuring all the settings are correct. I ignore the GUID. VS will change the GUID as soon as I save the project after adding it to solution.

I do a lot of DLL/C/C++ development and I have deviated from the standard project settings extensively over the years (particularly in regard to optimization, code generation etc) and I find the copy technique a quicker, surer way of setting up a project.

The project guids in the csproj file should be unique. From my experience if two csproj files are included in the same solution visual studio automatically regenerates one of the guids.