How to obtain primary key of table

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.NET

SQL-Server

.NET4

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.