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Is it possible to create a Windows Form in a C# Class Library?

I've been building DLL class libraries in C#, used as add-ons to an application which provides a Custom API. Up until now they've included mostly interfacing with databases, calculations, disk operations and so forth. I'm curious to know if I can build and display a Windows Form, displaying text boxes, buttons and so forth, inside a DLL Class Library?
I tried:
using System.Windows.Forms;
But that namespace is not recognized.
Thanks for the input.

What I find works best for me is to create a new Windows Forms project, and then go to the project properties and change it to a class library. This way, you can right click on folders in the solution explorer and all the WinForms items appear just as if it were still a WinForms project, but it's a class library. This also works with WPF applications.

You can definitely use Windows Forms inside your class library. There are few scenarios:
You are adding a form to the library using FormDesigner. You can right click on project name, click on Add and then in Windows form. It should add required References for your form
You are copying a form from other project. In this case the Visual Studio will not be able to identify the form and will show it as simple C# source file. In this case right click on References under the project in Visual Studio. Click on Add References and select Framework from left pane. Select System.Windows.Form and System.Drawing and click Ok. This should make the form understandable to Visual Studio and you can design it edit it using Form Designer.
If you are creating form using code, add the references to required assemblies as mentioned in the step 2. You should be able to use System.Windows.Form in your project with using System.Windows.Form; statement.

Yes, you can. You need to add a reference to System.Windows.Forms in the class library project (right-click on project -> Add -> Reference)

You can manually edit the project file to include windows forms:
Right click the project file --> "Edit Project File" and then under the <PropertyGroup> tag, add "<UseWindowsForms>true</UseWindowsForms>"
<PropertyGroup>
<TargetFramework>net472</TargetFramework>
<UseWindowsForms>true</UseWindowsForms>
<OutputType>Library</OutputType>
</PropertyGroup>

It's 2021-7-29 and I'm using VS2019 and net5.
When creating a new project, there is a template named "Windows Form Class Library", with App.WindowsForm in it's dependencies as default.
God and Microsofters heard our pray.

if the New form is already in a Windows project that has to stay windows
application..
try these Steps not sure if this is what you want... hope it helps.
Create new Dll Project
Copy Form.cs, FormDesigner.cs and form.resx
into the new dll project Folder
Add Existing

C# using others code

Iv'e downloaded a C# interval tree collection class class from here http://intervaltree.codeplex.com/SourceControl/list/changesets -> Right hand side -> Download.
However I can't open the whole project on my Microsoft Visual C# 2010 Express (that also runs C# XNA) because
Solution folders are not supported in this version of the application
Also I just want the class to use separately in my own seprate project.
I tried to copy the three important seeming files Interval.cs, IntervalNode.cs and IntervalTree.cs into my project but this generated the compile error
There are no importers which handle this file type
I've also tried to copy and paste the contents of the three files into my project, encapsulating them into there own namespace as well as there was a lot of code. I had to rearange some of the usings a little but have run into the problem that possibly it wants PowerCollections .dll and .pcb files as using Wintellect.PowerCollections; causes
The type or namespace name 'Wintellect' could not be found (are you missing a using directive or an assembly reference?)
I'm not sure how to continue or if I'm doing the right thing at all in how to get this class to work.

Add the library to your solution
Copy the IntervalTreeLib directory into your solution directory. Then, right-click your solution, and add existing project. Point it at IntervalTreeLib.csproj in IntervalTreeLib, and click Open. That should add the IntervalTreeLib project to your solution.
Add a reference to the library in your project
Then, in your project, add a reference to the IntervalTreeLib proejct:
- Right click the References folder, and Add Reference. Click the Projects tab, and select IntervalTreeLib.
Use the classes in your code
To use classes from the library in your source then, you need to either add:
using IntervalTreeLib;
void Foo() {
IntervalTree<int, int> tree = new ...
}
Or, refer to them by their full name:
IntervalTreeLib.IntervalTree<int, int> tree = new ...

Open the IntervalTreeLib.csproj file if you want to be able to open the project in it's entirety (or in your current solution add an existing project (you can right-click on the solution) and select the IntervalTreeLib.csproj). If you are trying to grab just the code file in your project, ensure you also grab the PowerCollections.dll file (I see it is in the same folder as the code files) or your code will not compile (as you have discovered). You'll need to add a reference to it and include the needed using statement at the top of the code files making use of this library (or use fully qualified name with the namespace).
using IntervalTreeLib;
or
var myObj = new IntervalTreeLib.[WhateverClass](...);
Also, make sure you read the license.txt file. You may need to include it if you are using the code. Give credit where it is due.
UPDATE:
If the test project is causing you problems, just open the library project. Ideally you could just open that and compile it, adding the output DLL files that are generated directly into your solution. This is ideal unless you are planning on changing the library source code itself.

Add the library to the references of the project you want to use it.

Since discussing that you are able to build Intervallib.dll, we will discuss about how you should the dll in your project.
Now in your proj, right click on the references part and add the dll intervallib.dll to your references. In your game.cs file, have the reference to the namespace as -- using IntervalTreeLib;
then you should actually copy the dll powercollections.dll to the bin directory of proj directory also.
you should copy this dll because there is an indirect link to the dll as it is used in IntervalTreeLib.dll
following these steps, I was able to execute this project.

How to use a DLL written in C# in other asp.net project?

I have a file named common.cs which contains commonly used functions like this
public int GetColumnIndexByNameTemplateField(GridViewRow row, string SearchColumnName)
{
int columnIndex = 0;
foreach (DataControlFieldCell cell in row.Cells)
{
if (cell.ContainingField is TemplateField)
{
if (((TemplateField)cell.ContainingField).HeaderText.Equals(SearchColumnName))
{
break;
}
}
columnIndex++;
}
return columnIndex;
}
Now I want to make that common.cs file change into common.dll, I created another class library project in which I copied all the code from common.cs file, added necessary references and built it to generate dll file, now I pasted that dll file from project debug directory into the bin directory of my asp.net project which is already using common.cs but whenever I try to import that dll using.
[DllImport("common.dll")]
public static extern String NewString(String x);
where NewString is another function, I get this error.
Unable to find an entry point named 'NewString' in DLL 'common.dll'.
I have checked already posted questions on stackoverflow as well as other guides on internet that tell how to make and use a dll but I don't know why I am getting this error also most of the guides show dll written in C++ where as my dll is written in C#, they also tell to use __declspec(dllexport) which tells that function is ready to be exported or something like that but I can't use this in C# dll I suppose.
I think its a basic question but this is my first time creating dll in C# so please help me out.
Thanks.

Ahmed if you want to turn any project into a library (.dll), double click the 'properties' folder in the solution explorer. Click the drop-down for project type and select 'Class Library' then click build, rebuild solution to recompile into a .dll.
Note that you can create a new ASP.NET project in the same solution by right clicking the solution and clicking 'add project'. Or you can open a new instance of Visual Studio. Once you open the ASP.NET project right click on references and click add reference. Click the browse tab, navigate to the folder containing the .dll and add it. Don't forget that any classes defined in the .dll must be marked as public or they will not be visible in other assemblies. Also don't forget the using statements for the namespaces that your classes are defined in.
When you click the 'Run Button' (the green arrow) it will always run the project that is in bold in the solution explorer. If a .dll is bolded then you will get an error that you cannot run a class library but you must instead change it to console application. If it is a console application then it must have a static void Main() function defined so it knows where to start. You can set any project as the startup project by right clicking it in solution explorer and click 'set as startup project'.
If you wish to run a program in your solution, but do not want to set it as the start up program than you can right click, go to debug and run. Organizing like-projects in a single solution can make maintaining them much easier.
Hope this helps.

You don't need to use DllImport. Since common.dll is a managed assembly, you simply need to add it as a reference in the ASP.NET project that's trying to use NewString.

if your assembly is written in c#, there's no need for a dllimport. rather go for add references like so - msdn entry (go for browse)

Paste the dll in the Bin folder of the site and you can use this by "using " at the top of the class of file then you will get all the methods of dll.

I can't add a using reference on my C# application

I'm using Visual Studio 2010 Beta2 and I can't add a reference to:
system.drawing
I've tried right click on the solution explorer but I can't find it there.
What gives?

Here is all you need to do in VS 2010:
In Solution Explorer right click on the References folder of your project
Select Add Reference.
Under the Assemblies category there is a sub category of Framework. If that is selected you should see a list of all of the Microsoft .NET Framework libraries
Find System.Drawing and double click it.
Then your project will have the reference added to it!

In VS2010 Beta 2 the references list may not be automatically sorted because it loads asynchronously. You may need to wait for it to finish loading then click the column header to sort.

Solution explorer is for adding references to external libraries.
It sounds like you just want to add the line
using System.Drawing;
to the top of one particular file.

what namespaces are needed for this piece of code? (c#)

Image Dummy = Image.FromFile("image.png");
Dummy.Save("image.bmp", ImageFormat.Bmp);
what the question says
i have these
using System;
using System.Drawing;
using System.Drawing.Imaging;
using System.Drawing.Drawing2D;
but i get an error saying that the namespace drawing does not exist in the namespace system :/

Add a reference to System.Drawing
When you use using statements, this lets the compiler know that when you say Image you really mean System.Drawing.Image for example.
However, now the compiler needs to know where System.Drawing is. By default, in Visual Studio, you will probably already reference System, System.Data, and System.Xml.
Now you are writing for System.Drawing. Right-click the project in the project browser, and select "Add reference...". This will present you with a tabbed interface that lets you select one of:
A .NET Reference. Any assembly in the GAC will be listed here. Scroll down and select System.Drawing for example.
COM Reference. For interfacing with non-.NET, yet very Windows components.
Projects. A Visual Studio nicety. Reference a DLL that has not been built yet. Select a project within the same solution. Intellisense before you compile.
Browse (for a file). If a .NET component or other type with exported definitions has already been built, you can reference the DLL from here.

Adding the "using" is only part of the equation. You also have to reference the DLL which contains the code you want to use.
For example, if you write a cool library under the namespace com.shuttleu.awesomelib, I could have the following at the top of my C# file:
using com.shuttleu.awesomelib;
On its own, that won't do me any good, I have to reference the DLL (referred to as an "assembly") in my project. You do that by right-clicking the project and selecting "Add Reference..."

In addition to the using statement, you also need to add the System.Drawing DLL (called an 'assembly') as a reference in your Visual Studio project. To do that, in the Solution Explorer pane usually at the right (if it's not open, go to View --> Solution Explorer), right click References and click Add Reference. In the .NET tab (open by default), scroll down until you see System.Drawing, select it, and click OK. After doing that, System.Drawing should appear under References in the Solution Explorer, and the using statement will now point to the correct DLL, therefore your code will work. :)

"Adding the "using" is only part of the equation. You also have to reference the DLL which contains the code you want to use"
Just curious--why do we have to do both for a console app, while in an VB.Net web app, the text "Imports System.Drawing" is sufficient?