How to obtain primary key of table

Question

Hi,
I want to know column which has primary key defined in sql server database.
Following is my code 

        SqlConnection con = new SqlConnection("Initial Catalog=Dbname; Data Source=MySqlServer; User=testuser; Password=pass123;");

        DataSet ds = new DataSet();

        SqlCommand cmd = new SqlCommand("select * from TableName", con);

        cmd.CommandType = CommandType.Text;

        SqlDataAdapter da = new SqlDataAdapter(cmd);

        da.Fill(ds);

        DataColumn[] columns;

        columns = ds.Tables[0].PrimaryKey;

        // Get the number of elements in the array.

        Response.Write("Column Count: " + columns.Length);

        for (int i = 0; i < columns.Length; i++)

        {

            Response.Write(columns[i].ColumnName + columns[i].DataType);

        }

In fact, i have defined one primary key on a column of TableName
But my code returns zero count.
please correct me, if i am wrong and provide me alternative solution
thank is advance.