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

Retrieving the Default Application for a File Type in Windows

To determine the default application for opening files of a specific type, you can utilize the Windows Registry. In HKEY_CLASSES_ROOT, each file extension has a corresponding key. The default application is stored in the Default value of this key.

Consider the following example for opening HTML files:

• Navigate to HKEY_CLASSES_ROOT.html
• The Default value will contain the registry key for the default application, e.g., HTMLFile

This method, however, has limitations. It relies on the registry's implementation, which may differ across Windows versions. A more reliable approach involves the Win32 API function AssocQueryString.

AssocQueryString

AssocQueryString allows you to query the default application for a specific file type.

Syntax:

uint AssocQueryString(
    AssocF flags, 
    AssocStr str,  
    string pszAssoc, 
    string pszExtra, 
    [Out] StringBuilder pszOut, 
    ref uint pcchOut
); 

Flags:

• AssocF.None: Use default settings
• AssocF.Open_ByExeName: Query for the default application by its executable name

Str:

• AssocStr.Command: Retrieve the command used to open files with the specified extension

Example:

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
);

AssocF flags = AssocF.Open_ByExeName;
AssocStr str = AssocStr.Command;
StringBuilder sb = new StringBuilder();
uint pcchOut = 0;

uint ret = AssocQueryString(flags, str, ".txt", null, sb, ref pcchOut);
if (ret == 0)
{
    Console.WriteLine($"Default text editor: {sb.ToString()}");
}
else
{
    Console.WriteLine("Error retrieving default text editor.");
}

This approach provides a more reliable way to determine the default application for a file type in Windows.

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