C# Signature:

[DllImport("dmcl40.dll", EntryPoint="dmAPIExec", CharSet=CharSet.Ansi, ExactSpelling=true,CallingConvention=CallingConvention.Cdecl)]
private extern static int dmAPIExec(string command);    

Sample Code:

public  bool Exec(string command, params object[] args)
{
    string cmd = String.Format(null,command, args);
    int res = dmAPIExec(cmd);
    return (res == 1);
}

Alternative Managed API:

Partially complete... http://workspaces.gotdotnet.com/dm