How Can I Programmatically Find the Default Application for a Specific File Type in Windows?

Finding the Default Application for File Types in Windows

Developers often need to find the default application associated with a specific file type. This is important for tasks such as invoking the default editor for a particular file extension.

To achieve this, one might consider using System.Diagnostics.Process.Start to open a file in its default application. However, this approach is limited and doesn't allow for the opening of files with non-standard extensions.

The reliable solution involves using the Win32 API function AssocQueryString. This function enables querying the operating system for the default application associated with a file type.

Here's how you can leverage AssocQueryString in C#:

using System.Runtime.InteropServices;

[DllImport("Shlwapi.dll", CharSet = CharSet.Unicode)]
public static extern uint AssocQueryString(
    AssocF flags, 
    AssocStr str,  
    string pszAssoc, 
    string pszExtra, 
    [Out] StringBuilder pszOut, 
    ref uint pcchOut
); 

[Flags]
public enum AssocF
{
    // ...
}

public enum AssocStr
{
    // ...
}

public static string AssocQueryString(AssocStr association, string extension)
{
    const int S_OK = 0;
    const int S_FALSE = 1;

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

    var sb = new StringBuilder((int)length); 
    ret = AssocQueryString(AssocF.None, association, extension, null, sb, ref length);
    if (ret != S_OK)
    {
        throw new InvalidOperationException("Could not determine associated string"); 
    }

    return sb.ToString();
}

By calling AssocQueryString and specifying the desired association, you can retrieve the full path to the default application. This allows you to directly interact with the default editor or processor for a particular file type, even if the file doesn't have a standard extension.

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