SHQueryRecycleBin

C# Signature:

[DllImport("shell32.dll")]
static extern int SHQueryRecycleBin(string pszRootPath, ref SHQUERYRBINFO
   pSHQueryRBInfo);

User-Defined Types:

[StructLayout(LayoutKind.Sequential, Pack=4)]
public struct SHQUERYRBINFO
{
    public int  cbSize;
    public long i64Size;
    public long i64NumItems;
}

Sample Code:

public static int GetCount()
{
    SHQUERYRBINFO sqrbi = new SHQUERYRBINFO();
    sqrbi.cbSize = Marshal.SizeOf(typeof(SHQUERYRBINFO));
    int hresult = SHQueryRecycleBin(string.Empty, ref sqrbi);
    return (int)sqrbi.i64NumItems;
}

N.B.

int hresult = SHQueryRecycleBin(@"C:\", ref sqrbi);