How Can I Programmatically Find the Default Application for a File Type in .NET?

Finding the Default Application for Opening a File Type on Windows (.NET)

In .NET Framework 2.0 using C#, it's possible to open files in their default application by utilizing System.Diagnostics.Process.Start(). However, for files without specific extensions, determining the associated application can be tricky.

Using the Win32 API

To overcome this limitation, utilizing the Win32 API function AssocQueryString() is recommended. This function returns the string associated with a particular file type, allowing developers to extract the executable path of the default application.

The code snippet below demonstrates how to use AssocQueryString():

using System.Runtime.InteropServices;
using System.Text;

public static string GetDefaultApplicationForExtension(string extension)
{
    const int S_OK = 0;
    const int S_FALSE = 1;

    uint length = 0;
    uint ret = AssocQueryString(AssocF.None, AssocStr.Executable, extension, null, null, ref length);
    if (ret != S_FALSE)
    {
        throw new InvalidOperationException("Could not determine associated string");
    }

    StringBuilder sb = new StringBuilder((int)length); // (length-1) will probably work too as the marshaller adds null termination
    ret = AssocQueryString(AssocF.None, AssocStr.Executable, extension, null, sb, ref length);
    if (ret != S_OK)
    {
        throw new InvalidOperationException("Could not determine associated string"); 
    }

    return sb.ToString();
}

By invoking this function and providing the file extension as an argument, developers can obtain the default application path, enabling them to open files in the desired application programmatically.

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