How do I start a process from C#?

As suggested by Matt Hamilton, the quick approach where you have limited control over the process, is to use the static Start method on the System.Diagnostics.Process class...

using System.Diagnostics;
...
Process.Start("process.exe");

The alternative is to use an instance of the Process class. This allows much more control over the process including scheduling, the type of the window it will run in and, most usefully for me, the ability to wait for the process to finish.

using System.Diagnostics;
...
Process process = new Process();
// Configure the process using the StartInfo properties.
process.StartInfo.FileName = "process.exe";
process.StartInfo.Arguments = "-n";
process.StartInfo.WindowStyle = ProcessWindowStyle.Maximized;
process.Start();
process.WaitForExit();// Waits here for the process to exit.

This method allows far more control than I've mentioned.

You can use the System.Diagnostics.Process.Start method to start a process. You can even pass a URL as a string and it'll kick off the default browser.

Just as Matt says, use Process.Start.

You can pass a URL, or a document. They will be started by the registered application.

Example:

Process.Start("Test.Txt");

This will start Notepad.exe with Text.Txt loaded.

Use the Process class. The MSDN documentation has an example how to use it.

If using on Windows

Process process = new Process();
process.StartInfo.FileName = "Test.txt";
process.Start();

Works for .Net Framework but for Net core 3.1 also need to set UseShellExecute to true

Process process = new Process();
process.StartInfo.FileName = "Test.txt";
process.StartInfo.UseShellExecute = true;
process.Start();

You can use this syntax:

private void button1_Click(object sender, EventArgs e)  {
   System.Diagnostics.Process.Start(/*your file name goes here*/);
}

Or even this:

using System;
using System.Diagnostics;
//rest of the code

private void button1_Click(object sender, EventArgs e)  {
   Process.Start(/*your file name goes here*/);
}

Both methods would perform the same task.