Develop Reference

How to build a DLL with custom file extension in a C# project?

Is there any way to compile a C# DLL project (visual studio) into a file with a custom extension? for example plugin.cpx instead of plugin.dll (cpx = Custom Plugin eXtension!)
And can I use any extension I want?

I'm adding this as a new answer because it is just that. And it's a lot easier than my earlier offering, in many ways.
You can explicitly specify the target extension by manually editing the project's .csproj file.
In "Solution Explorer," right click on the project and select the "Unload project" command. Then, right-click again and select "Edit MyProject.csproj" (where "MyProject" would be the actual name, obviously).
To make the extension change apply to all configurations and/or platforms, add the following three lines, near the top of the file (just before the first of the other PropertyGroup entries):
<PropertyGroup>
<TargetExt>.cpx</TargetExt>
</PropertyGroup>
Then save the file, close it, and "Reload" the project (again, via a right-click in the Solution Explorer).
To change the extension only for a specific configuration or platform, add the TargetExt line inside the PropertyGroup relevant to that configuration.

And can I use any extension I want?
Yes, you can, but it's a bit 'tricky'.
For C# projects, you will need to add a custom "Post Build Event" to rename the output file. For a DLL project (as yours is) you can add a post-build command like that shown below, in the "Build Events" tab of the "Properties" window:
Note: Be sure to use the $(TargetName) macro, not $(TargetFileName), as the latter will implicitly include the .dll extension. But you could use:
rename "$(TargetDir)$(TargetFileName)" "$(TargetName).cpx"
EDIT: Also, you will need to tell the builder to delete any old version of the .cpx file, at some stage. This could be done as a pre-build event, as follows:
IF EXIST "$(TargetDir)$(TargetName).cpx" del "$(TargetDir)$(TargetName).cpx"
Without this, the attempt to rename the output file will fail (as the new 'target' will already exist).
EDIT-2: As noted in the comment by the OP, without the " quotes, the pre- and post-build commands will fail if there are spaces in the file paths.

Adding common files to different projects in C#

I have 2 project in difference paths. I want to use some common files (.cs files) in both of projects.
D:\Tests\WindowsFormsApplication_1\WindowsFormsApplication_1
D:\Tests\WindowsFormsApplication_2\WindowsFormsApplication_2
D:\Tests\Common
How do I add this common files from one path ("Common" folder) for both of the projects?

Yes, You can. Right Click on your project, "Add" -> "Existing Item...", select your *.cs file and click on Down arrow next to the "Add" button and select "Add As Link".

There is no way to use same files in 2 different project without copying them. But you can add the third project in type of class library and put your shared files in it then add references to that project in your previous projects

If you want the common files to be available to edit in your other projects then
follow user4015859's suggestion re setting up a class library project for your common files, but rather than adding a reference to that project in your other solutions, add the actual project to your solution by right clicking on the solution and select Add > Existing Project...
You still need to add a reference but in the reference Manager dialog (right click on references to open it) select Projects on the left hand side and your common project should be listed. Tick it and click OK.
To use the classes etc from the common project just add its namespace in the units where you want to use it.
This way you can edit the common files from either of your other solutions and you don't need to manually build your common solution when any of the files are changed.

Add compile parameter to csc command using Visual Studio IDE

The solution consists of two projects: main.csproj and helper.csproj.
What Id'like to do is using dll which helper project will be complied into, as an embedded resource for main.dll.
For that purposes it's seems resonable to add custom compile attribute for project main: /resource: <path to dll>.
The problem is I can't find how to add this compile parameter through the Project Property.
PS Maybe someone can suggest other solution to avoid making changes in compile process.

You should be able to add the helper assembly as a resource in the main.csproj. That will make MsBuild generate the correct parameters for csc.
(MsBuild is the build engine used by .NET in general up to and including 4.x and is also used by VisualStudio.)
What you can do to set this up is:
Right click the Main project in the Visual Studio solution explorer and select Add existing item. Add the assembly to be embedded as a linked item from the bin folder of the helper project. (click the little arrow on the Add button in the selection dialog to access the option to add as a link).
In the properties for the item in the Main project, set Action to Embedded resource.
Tricky bit would be to include the correct build so that you include the debug build or the release build depending on what configuration you are building.
If you need that, you can either:
edit main.csproj file to include the ${Configuration} variable in the path for the helper dll.
Add a pre-build step to the main.csproj file to copy in the assembly to a fixed place and include the file from there (the include as link bit is no longer needed then)
To make sure you always build the helper assembly when you build the main assembly I would recommend you add a project reference to the main project.

What is the best way to duplicate a .csproj (for creating the same app with different named exes)?

I have a solution in visual studio where one project (.csproj) is set to create an exe.
I would need to create a duplicate copy of this project so that I can name it something different and change the icon for it. (All of the rest of the projects can stay the same)
I am using conditional compilation symbols for that project, but I don't want to create a whole separate solution configuration because that requires expensive rebuilding of the entire solution.

In Visual Studio under Build -> Configuration Manager, you can create a new configuration for your project and clone it from your release build, then in your project properties you can customize it.
For the icon, you'll want to refer to Set a different ApplicationIcon for each build configuration

Create a copy of the project on disk (outside Visual Studio) and add the copied project to the solution. Then you can modify output assembly name, icon. etc..
However better practice would be to perform the necessary operations as postbuild step (e.g. batch/powershell script) as you will have to keep the projects synchronized (added/renamed/removed files, references...)

Copy it somewhere else and change the assembly name and namespace may be on the project property window( right click and property)

I dont know what your trying to accompish but possible solutions:
add post build event that will copy exe / (exe ad dll-s) to another directory
if you use TFS, edit your Build so it will create copys
Cheerz,

Combining between two csproj files safely

I have two very long and detailed .csproj files.
I want to combine them into one.
I used text comparer but the items are not in the same order
and it's hard to isolate differences.
How would you recommend to combine them?
(they have compile, include, post build and after build events)
edit:
I want to merge 2 unrelated projects with some common dependencies

Copy the files from project 1 into the project 2 folder. Then turn on "Show all files" so you can see the files that aren't in the project. Then right-click each file and choose "Include in project".

I would probably do most of this in Visual Studio. You can either drag the files you need from one project to the other, or you can copy all the files from one project folder to the other in the file system and turn on "show all files" in the solution explorer to show which need to be added. You will then need to align the namespaces. The Class View window can help identify types which do not fall inside the right namespace. A refactoring tool like Resharper can also help fix up the namespaces.
If you have explicit pre or post build events in each project, I would use the VS GUI to show these and manually combine. If you have custom build targets/tasks in the files, I would isolate these in a good text/XML editor and manually union them as required.
The other thing you will have to do is to add references to the final project which it did not originally require but were required by the other project. It should be quite quick to identify which references need to be added, by attempting compilation and inspecting any errors.