Parametrizing geometry sql command c# in ASP.NET, not working

I have used this string and tested it with string concatenation.But as you know it is not safe to use this to format an sql command.

 SqlCommand param = new SqlCommand();
        param.CommandText = "INSERT INTO Circle (Center_Point, Circle_Data) VALUES (geometry::STGeomFromText('POINT(@center_lat @center_lng)',0),geometry::STGeomFromText('POLYGON((@polygon))',0));";
        param.Parameters.Add(new SqlParameter("@center_lat", center_lat));
        param.Parameters.Add(new SqlParameter("@center_lng", center_lng));
        param.Parameters.Add(new SqlParameter("@polygon", polygon));

I go to parametrize the string and get the following error:

System.Data.SqlClient.SqlException (0x80131904): A .NET Framework error occurred during execution of user-defined routine or aggregate "geometry": System.FormatException: 24141: A number is expected at position 17 of the input. The input has @center_lat.

Looks like it hasn't put the value into the string. but when I step through the code it does indeed hold the value.

What could be the problem?

Thanks

Thanks to Me.Name. I had to add the correct assemblies to the ASP.net project, which enabled me to set the UDT type correctly. Updated Code is below.

SqlCommand param = new SqlCommand();
        SqlGeometry point = SqlGeometry.Point(center_lat,center_lng,0);
        SqlGeometry poly = SqlGeometry.STPolyFromText(new SqlChars(new SqlString(polygon)),0);
        param.CommandText = "INSERT INTO Circle (Center_Point, Circle_Data) VALUES (@point,@poly);";
        param.Parameters.Add(new SqlParameter("@point", SqlDbType.Udt));
        param.Parameters.Add(new SqlParameter("@poly", SqlDbType.Udt));
        param.Parameters["@point"].UdtTypeName = "geometry";
        param.Parameters["@poly"].UdtTypeName = "geometry";
        param.Parameters["@point"].Value = point;
        param.Parameters["@poly"].Value = poly;