How to get the full path of running process?

I guess you already have the process object of the running process (e.g. by GetProcessesByName()).
You can then get the executable file name by using:

Process p;
string filename = p.MainModule.FileName;

 using System.Diagnostics;
 var process = Process.GetCurrentProcess(); // Or whatever method you are using
 string fullPath = process.MainModule.FileName;
 //fullPath has the path to exe.

There is one catch with this API, if you are running this code in 32 bit application, you'll not be able to access 64-bit application paths, so you'd have to compile and run you app as 64-bit application (Project Properties → Build → Platform Target → x64).

What you can do is use WMI to get the paths. This will allow you to get the path regardless it's a 32-bit or 64-bit application. Here's an example demonstrating how you can get it:

// include the namespace
using System.Management;

var wmiQueryString = "SELECT ProcessId, ExecutablePath, CommandLine FROM Win32_Process";
using (var searcher = new ManagementObjectSearcher(wmiQueryString))
using (var results = searcher.Get())
{
    var query = from p in Process.GetProcesses()
                join mo in results.Cast<ManagementObject>()
                on p.Id equals (int)(uint)mo["ProcessId"]
                select new
                {
                    Process = p,
                    Path = (string)mo["ExecutablePath"],
                    CommandLine = (string)mo["CommandLine"],
                };
    foreach (var item in query)
    {
        // Do what you want with the Process, Path, and CommandLine
    }
}

Note that you'll have to reference the System.Management.dll assembly and use the System.Management namespace.

For more info on what other information you can grab out of these processes such as the command line used to start the program (CommandLine), see the Win32_Process class and WMI .NET for for more information.

A solution for:

• Both 32-bit AND 64-bit processes
• System.Diagnostics only (no System.Management)

I used the solution from Russell Gantman and rewritten it as an extension method you can use like this:

var process = Process.GetProcessesByName("explorer").First();
string path = process.GetMainModuleFileName();
// C:\Windows\explorer.exe

With this implementation:

internal static class Extensions {
    [DllImport("Kernel32.dll")]
    private static extern bool QueryFullProcessImageName([In] IntPtr hProcess, [In] uint dwFlags, [Out] StringBuilder lpExeName, [In, Out] ref uint lpdwSize);

    public static string GetMainModuleFileName(this Process process, int buffer = 1024) {
        var fileNameBuilder = new StringBuilder(buffer);
        uint bufferLength = (uint)fileNameBuilder.Capacity + 1;
        return QueryFullProcessImageName(process.Handle, 0, fileNameBuilder, ref bufferLength) ?
            fileNameBuilder.ToString() :
            null;
    }
}