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DataTable.PrimaryKey Property

Gets or sets an array of columns that function as primary keys for the data table.

[Visual Basic]
Public Property PrimaryKey As DataColumn ()
[C#]
public DataColumn[] PrimaryKey {get; set;}
[C++]
public: __property DataColumn* get_PrimaryKey();
public: __property void set_PrimaryKey(DataColumn*[]);
[JScript]
public function get PrimaryKey() : DataColumn[];
public function set PrimaryKey(DataColumn[]);

Property Value

An array of DataColumn objects.

Exceptions

Exception Type Condition
DataException The key is a foreign key.

Remarks

The primary key of a table must be unique to identify the record in the table. It's also possible to have a table with a primary key made up of two or more columns. This occurs when a single column can't contain enough unique values. For example, a two column primary key might consist of a "FirstName" and "LastName" column. Because primary keys can be made up of more than one column, the PrimaryKey property consists of an array of DataColumn objects.

Example

[Visual Basic, C#, C++] The first example shows how to return the primary key columns for a DataTable displayed in a DataGrid. The second example demonstrates how to set the primary key columns for a DataTable.

[Visual Basic] 
Private Sub GetPrimaryKeys(myTable As DataTable)
   ' Create the array for the columns.
   Dim colArr() As DataColumn 
   colArr = myTable.PrimaryKey
   ' Get the number of elements in the array.
   Console.WriteLine("Column Count: " & colArr.Length.ToString())
   Dim i As Integer
   For i = 0 To colArr.GetUpperBound(0)
      Console.WriteLine(colArr(i).ColumnName & colArr(i).DataType.ToString())
   Next i
End Sub

Private Sub SetPrimaryKeys()
   ' Create a new DataTable and set two DataColumn objects as primary keys.
   Dim myTable As DataTable = new DataTable()
   Dim keys(2) As DataColumn
   Dim myColumn  As DataColumn
   ' Create column 1.
   myColumn = New DataColumn()
   myColumn.DataType = System.Type.GetType("System.String")
   myColumn.ColumnName= "FirstName"
   ' Add the column to the DataTable.Columns collection.
   myTable.Columns.Add(myColumn)
   ' Add the column to the array.
   keys(0) = myColumn

   ' Create column 2 and add it to the array.
   myColumn = New DataColumn()
   myColumn.DataType = System.Type.GetType("System.String")
   myColumn.ColumnName = "LastName"
   myTable.Columns.Add(myColumn)
   ' Add the column to the array.
   keys(1) = myColumn
   ' Set the PrimaryKeys property to the array.
   myTable.PrimaryKey = keys
End Sub

[C#] 
private void GetPrimaryKeys(DataTable myTable){
   // Create the array for the columns.
   DataColumn[] colArr;
   colArr = myTable.PrimaryKey;
   // Get the number of elements in the array.
   Console.WriteLine("Column Count: " + colArr.Length);
   for(int i = 0; i < colArr.Length; i++){
      Console.WriteLine(colArr[i].ColumnName + colArr[i].DataType);
   }
}

private void SetPrimaryKeys(){
   // Create a new DataTable and set two DataColumn objects as primary keys.
   DataTable myTable = new DataTable();
   DataColumn[] keys = new DataColumn[2];
   DataColumn myColumn;
   // Create column 1.
   myColumn = new DataColumn();
   myColumn.DataType = System.Type.GetType("System.String");
   myColumn.ColumnName= "FirstName";
   // Add the column to the DataTable.Columns collection.
   myTable.Columns.Add(myColumn);
   // Add the column to the array.
   keys[0] = myColumn;

   // Create column 2 and add it to the array.
   myColumn = new DataColumn();
   myColumn.DataType = System.Type.GetType("System.String");
   myColumn.ColumnName = "LastName";
   myTable.Columns.Add(myColumn);
   // Add the column to the array.
   keys[1] = myColumn;
   // Set the PrimaryKeys property to the array.
   myTable.PrimaryKey = keys;
}

[C++] 
private:
 void GetPrimaryKeys(DataTable* myTable){
    // Create the array for the columns.
    DataColumn* colArr[];
    colArr = myTable->PrimaryKey;
    // Get the number of elements in the array.
    Console::WriteLine(S"Column Count: {0}", __box(colArr->Length));
    for(int i = 0; i < colArr->Length; i++){
       Console::WriteLine(S"{0}{1}", colArr[i]->ColumnName, colArr[i]->DataType);
    }
 }
 
private:
 void SetPrimaryKeys(){
    // Create a new DataTable and set two DataColumn objects as primary keys.
    DataTable* myTable = new DataTable();
    DataColumn* keys[] = new DataColumn*[2];
    DataColumn* myColumn;
    // Create column 1.
    myColumn = new DataColumn();
    myColumn->DataType = System::Type::GetType(S"System.String");
    myColumn->ColumnName= S"FirstName";
    // Add the column to the DataTable.Columns collection.
    myTable->Columns->Add(myColumn);
    // Add the column to the array.
    keys[0] = myColumn;
 
    // Create column 2 and add it to the array.
    myColumn = new DataColumn();
    myColumn->DataType = System::Type::GetType(S"System.String");
    myColumn->ColumnName = S"LastName";
    myTable->Columns->Add(myColumn);
    // Add the column to the array.
    keys[1] = myColumn;
    // Set the PrimaryKeys property to the array.
    myTable->PrimaryKey = keys;
 }

[JScript] No example is available for JScript. To view a Visual Basic, C#, or C++ example, click the Language Filter button Language Filter in the upper-left corner of the page.

Requirements

Platforms: Windows 98, Windows NT 4.0, Windows Millennium Edition, Windows 2000, Windows XP Home Edition, Windows XP Professional, Windows Server 2003 family, .NET Compact Framework

See Also

DataTable Class | DataTable Members | System.Data Namespace | DataColumn | PrimaryKey | DataColumnCollection

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