How to obtain primary key of table

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.