Forms' Does Not Exist in the Namespace System.Windows

Forms' does not exist in the namespace system.windows

Expand the project in Solution Tree, Right-Click on References, Add Reference, Select System.Windows.Forms on Framework tab.

You need to add reference to some non-default assemblies sometimes.

From comments: for people looking for VS 2019+: Now adding project references is Right-Click on Dependencies in Solution Explorer.

For people looking for VS Code: How do I add assembly references in Visual Studio Code

Forms' does not exist in the namespace 'System.Windows

Did you remember to add the reference thusly:

Because if you didn't that might very well be why, you need to right click on references, that is hightlighted in blue on the image. and then select "Add reference".

I did this, and I am able to create and display a form from a console app.

EDIT:

How to add assembly references in Visual Studio Code?

Please refer to this link for this visual-studio-code solution. I apoligize. However, the other solution applies just fine for visual studio :) So I am leaving it in :)

I does actually seem like that System.Windows.Forms is not supported for VS Code, due to a subset of Windows.Forms require some windows specific features.

For the full feature support, the recommended method is to install Visual Studio community.

I apoligize for the confusion. I should have checked first, if the library was even supported. I didn't.

https://code.visualstudio.com/docs/languages/csharp

The link at the bottom has a little FAQ section that tell you to install visual studio community, for full feature support.

The type or namespace name 'Forms' does not exist in the namespace 'System.Windows'

Include the System.Windows.Forms assembly in your project (as the error message suggested).

Go to your Solution Explorer, right click your project. Then click Add Reference. Choose System.Windows.Forms and hit OK.

The type or namespace Windows does not exists in namespace System when compiling code dynamically c#

Add a namespace to it, matching your main project's namespace. I believe it's executing in a separate context since it doesn't have the same namespace, so it doesn't find the references included in the rest of the project.

Also, change your compiler options to

                    new System.CodeDom.Compiler.CompilerParameters()
                    {
                        GenerateInMemory = true,
                        CoreAssemblyFileName = "mscorlib.dll",
                        ReferencedAssemblies = { "System.dll", "System.Windows.dll", "System.Windows.Forms.dll","Microsoft.CSharp.dll" }
                    }

that works, I've tried it. You may not need all of those referenced assemblies, its the CoreAssemblyFileName that really mattered.

Can't use System.Windows.Forms

A console application does not automatically add a reference to System.Windows.Forms.dll.

Right-click your project in Solution Explorer and select Add reference... and then find System.Windows.Forms and add it.

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