Usually you have to dig through SQL references to find out how to get metadata information for specific DBMS. There are few servers that support same SQL commands. Sometimes the syntax differs slightly, sometimes a server does not support certain statement. Now you can forget about those problems because dotConnect for SQL Server retrieves the metadata for you.

With dotConnect for SQL Server you can take advantage of a very useful feature - GetSchema method. It allows you to read server schema information without writing queries and parsing the output. All information you may want to obtain is brought to you by single function in easy-to-process format. You can get information on databases, tables, columns, indexes, users, stored procedures and functions, user-defined procedures, and reserved words. The method is introduced in System.Data.Common.DbConnection.

To illustrate capabilities of the feature, we have prepared MetaData sample. Please refer to the sample to learn the functionality in a quick glance.

This article consists of the following sections:

• How To Use
• GetSchema Method Reference
• Samples

How To Use

GetSchema method is available in three overloads, each of them serves its own purpose. All overloads return System.Data.DataTable object that contains information about server elements.

public virtual abstract DataTable GetSchema();

Overloads Public Overridable MustOverride Function GetSchema() As DataTable

If you call the GetSchema method without parameters, or with single parameter "MetaDataCollections" (which is actually the same), the table object returned by the method will contain three columns. The first field of every row is a keyword allowed to be passed to the method (as collectionName argument). The second field is the number of restriction values for this keywords (passed through restrictionValues argument). The third field is not used in dotConnect for SQL Server. It is always zero.

public virtual abstract DataTable GetSchema(
   string collectionName
);

Overloads Public Overridable MustOverride Function GetSchema( _
   ByVal collectionName As String _
) As DataTable

GetSchema with 1 argument returns general information about the collection queried. For example, GetSchema("Users") returns list of users on the server.

public virtual abstract DataTable GetSchema(
   string collectionName,
   string[] restrictionValues
);

Overloads Public Overridable MustOverride Function GetSchema( _
   ByVal collectionName As String, _
   ByVal restrictionValues() As String _
) As DataTable

In this overload first parameter is name of a collection, and second parameter is the array of restrictions to be applied when querying information. These restrictions specify which subset of the collection will be returned. The restrictions can include, for example, the database name (in this case, only collection elements belonging to this database will be returned) or the mask for the name of collection elements (only the elements satisfying this mask will be returned). The quantity and description of restrictions allowed for each metadata collection are represented in the table here. Their number can also be obtained from the return of the GetSchema() method. If the second parameter is null/Nothing, it is ignored.

GetSchema Method Reference

The following table provides detailed information on metadata collections that can be retrieved using the GetSchema method, and restrictions that can be applied for them. When calling the GetSchema method, you can pass all or few arguments. In the latter case, some default values are assumed, if they were not specified explicitly. The default value of database restriction is , table restriction is base table (.NET2) or table (.NET1). Some collections are not supported in older server versions. If you try ro get metadata for unsupported collection you will get exception with message "Collection not defined".

Samples

The following code fragment is an elegant way to detect the existence of a table in the Test database.

string tableName = "dept";
if (myDbConnection.GetSchema("Tables", new string[] { "Test", tableName }).Rows.Count > 0)
{
Console.WriteLine("Table " + tableName + " exists in the database.");
}

Dim tableName As String = "dept"
Dim restrictions() As String = {"Test", tableName}
If (myDbConnection.GetSchema("Tables", restrictions).Rows.Count > 0) Then
  Console.WriteLine("Table " + tableName + " exists in the database.")
End If

The next sample shows how to retrieve columns information from a table and render it to console.

static void GetTableInfo(SqlConnection myDbConnection, string tableName)
{
  myDbConnection.Open();
  DataTable myDataTable = myDbConnection.GetSchema("Columns", +
  new string[] { "Test", tableName });
  for (int i = 0; i < myDataTable.Columns.Count; i++)
  {
    Console.Write(myDataTable.Columns[i].Caption + "\t");
  }
  Console.WriteLine();
  foreach (DataRow myRow in myDataTable.Rows)
  {
    foreach (DataColumn myCol in myDataTable.Columns)
    {
      Console.Write(myRow[myCol] + "\t");
    }
    Console.WriteLine();
  }
  myDbConnection.Close();
}

Public Sub GetTableInfo(ByVal myDbConnection As SqlConnection, ByVal tableName As String)
  myDbConnection.Open()
  Dim restrictions() As String = {"Test", tableName}
  Dim myDataTable As DataTable = myDbConnection.GetSchema("Columns", restrictions)
  Dim i As Int32
  For i = 0 To myDataTable.Columns.Count - 1
    Console.Write(myDataTable.Columns(i).Caption & Chr(9))
  Next
  Console.WriteLine()
  Dim myRow As DataRow
  Dim myCol As DataColumn
  For Each myRow In myDataTable.Rows
    For Each myCol In myDataTable.Columns
      Console.Write(myRow(myCol) & Chr(9))
    Next
    Console.WriteLine()
  Next
  myDbConnection.Close()
End Sub

The following sample demonstrates how to generate SQL CREATE TABLE statement basing on metadata retrieved with GetSchema method. The generated script will work with all database management systems that support ANSI standard. Only column name and type are included in the script.

static void GetCreateTable(SqlConnection myDbConnection, string tableName)
{
  //Open the connection
  myDbConnection.Open();
  //Fill DataTable with columns information
  DataTable myDataTable = myDbConnection.GetSchema("Columns", +
  new string[] { "Test", tableName });
  string queryText = "CREATE TABLE " + tableName + " (\n";
  string fieldLine;
  DataRow myRow;
  //For every row in the table
  for (int i = 0; i < myDataTable.Rows.Count; i++)
  {
    //Get column name and type
    myRow = myDataTable.Rows[i];
    fieldLine = myRow[0] + " " + myRow[1];
    //Add comma or closing bracket
    if (i < myDataTable.Rows.Count - 1)
    {
      fieldLine = fieldLine + ",\n";
    }
    else
    {
      fieldLine = fieldLine + ")";
    }
    //Add new column to script
    queryText = queryText + fieldLine;
  }
  Console.WriteLine(queryText);
  //Close the connection
  myDbConnection.Close();
}

Public Sub GetCreateTable(ByVal myDbConnection As SqlConnection, ByVal tableName As String)
  'Open the connection
  myDbConnection.Open()
  Dim restrictions() As String = {"Test", tableName}
  'Fill DataTable with columns information
  Dim myDataTable As DataTable = myDbConnection.GetSchema("Columns", restrictions)
  Dim queryText As String = "CREATE TABLE " + tableName + " (" + System.Environment.NewLine
  Dim fieldLine As String
  Dim myRow As DataRow
  Dim i As Int32
  'For every row in the table
  For i = 0 To myDataTable.Rows.Count - 1
    'Get column name and type
    myRow = myDataTable.Rows(i)
    fieldLine = myRow(0) & " " & myRow(1)
    'Add comma or closing bracket
    If (i < myDataTable.Rows.Count - 1) Then
      fieldLine = fieldLine + "," + System.Environment.NewLine
    Else
      fieldLine = fieldLine + ")"
    End If
    'Add new column to script
    queryText = queryText + fieldLine
  Next
  Console.WriteLine(queryText)
  'Close the connection
  myDbConnection.Close()
End Sub

Also you can get a metadata of query result set using the GetSchemaTable method of SqlDataReader and the ShemaTable property of SqlDataTable classes.

See Also

SqlConnection Class | DbConnectionBase.GetSchema Method | dotConnect for SQL Server Articles