How to Copy the Contents of a String to the Clipboard in C#

How do I copy the contents of a String to the clipboard in C#?

You can use System.Windows.Forms.Clipboard.SetText(...).

How to copy data to clipboard in C#

There are two classes that lives in different assemblies and different namespaces.

WinForms: use following namespace declaration, make sure Main is marked with [STAThread] attribute:
```
using System.Windows.Forms;
```
WPF: use following namespace declaration
```
using System.Windows;
```
console: add reference to System.Windows.Forms, use following namespace declaration, make sure Main is marked with [STAThread] attribute. Step-by-step guide in another answer
```
using System.Windows.Forms;
```

To copy an exact string (literal in this case):

Clipboard.SetText("Hello, clipboard");

To copy the contents of a textbox either use TextBox.Copy() or get text first and then set clipboard value:

Clipboard.SetText(txtClipboard.Text);

See here for an example.
Or... Official MSDN documentation or Here for WPF.

Remarks:

Clipboard is desktop UI concept, trying to set it in server side code like ASP.Net will only set value on the server and has no impact on what user can see in they browser. While linked answer lets one to run Clipboard access code server side with SetApartmentState it is unlikely what you want to achieve.
If after following information in this question code still gets an exception see "Current thread must be set to single thread apartment (STA)" error in copy string to clipboard
This question/answer covers regular .NET, for .NET Core see - .Net Core - copy to clipboard?

copy text in clipboard

You should use the Clipboard class (msdn) and SelectedText property of TextBox (msdn):

To copy data to the clipboard use the SetText method (msdn).

To get data from the clipboard use the GetText method (msdn).

The following code allows you to copy selected text from TextBox to clipboard:

Clipboard.SetText(tbText.SelectedText);

How to get content of a text file and copy it to clipboard?

You need a reference to System.Windows or System.Windows.Forms

var content = File.ReadAllText("filepath.txt");
Clipboard.SetText(content);

How to copy data in a text file to the clipboard?

if you want to copy the content of a text file to Clipboard the use the following code:

Clipboard.SetText(File.ReadAllText("your file path"));

(Original answer, before question edit: If you want to copy the content of a notepad to Clipboard, then just select the text and press ctrl+c. it will copy the text to the Clipboard.)

How do I copy text to the clipboard with partial italics?

It seems you have to insert a 'header' in the html string, I found 2 examples for this:

MSDN blog
StackOverFlow

This works with Word:

Example code:

    Clipboard.SetText(@"Version:1.0
                        StartHTML:000125
                        EndHTML:000260
                        StartFragment:000209
                        EndFragment:000222
                        <HTML>
                        <head>
                        <title>HTML clipboard</title>
                        </head>
                        <body>
                        <!–StartFragment–><b>Hello!</b><!–EndFragment–>
                        </body>
                        </html>", 
                        TextDataFormat.Html);

This copies a Hello! to your clipboard, you have to change the fragments based on the size I think so I don't know exactly how that would work with a dynamic string but I hope this will get you started. 666

If you can also use RTF

Clipboard.SetText(@"{\rtf1\ansi This is in \i\f0\fs17 italic\i0.}",
                    TextDataFormat.Rtf);

Example with string

var q = "test123";
Clipboard.SetText(@"{\rtf1\ansi This is in \i\f0\fs17 " + q + @"\i0.}",
                    TextDataFormat.Rtf);

var q = "test123";
Clipboard.SetText( $@"{\rtf1\ansi This is in \i\f0\fs17 {q}\i0.}",
                    TextDataFormat.Rtf);

Note the @ before the 2nd part of the string, if you need to escape certain characters (you will need that with RTF) add @ in front of each opening ".

This seems to be a lot easier because you don't have to insert the header but the formatting itself is abit more complicated imo.