QueryColumnMetadata Class
Represents the metadata for a column within a query result.
Inheritance Hierarchy
System.Object
Microsoft.Web.Management.DatabaseManager.QueryColumnMetadata
Namespace: Microsoft.Web.Management.DatabaseManager
Assembly: Microsoft.Web.Management.DatabaseManager (in Microsoft.Web.Management.DatabaseManager.dll)
Syntax
'Declaration
Public Class QueryColumnMetadata
'Usage
Dim instance As QueryColumnMetadata
public class QueryColumnMetadata
public ref class QueryColumnMetadata
public class QueryColumnMetadata
The QueryColumnMetadata type exposes the following members.
Constructors
| Name | Description | |
|---|---|---|
.gif "Public method") |
QueryColumnMetadata | Creates a new instance of the QueryColumnMetadata class. |
Top
Properties
| Name | Description | |
|---|---|---|
.gif "Public property") |
ColumnWidth | Gets or sets the column width for the column metadata within a query result. |
|
IsComputed | Gets or sets a value that indicates whether the column data is computed within a query result. |
|
IsNullable | Gets or sets a value that indicates whether the column data supports nulla null reference (Nothing in Visual Basic) within a query result. |
|
Name | Gets or sets the column name for the column metadata within a query result. |
|
Tag | Returns a generic object that is stored in the QueryColumnMetadata class. |
|
TypeInfo | Retrieves the type information for the query column's metadata. |
Top
Methods
| Name | Description | |
|---|---|---|
|
Equals | (Inherited from Object.) |
.gif "Protected method") |
Finalize | (Inherited from Object.) |
|
GetHashCode | (Inherited from Object.) |
|
GetType | (Inherited from Object.) |
|
MemberwiseClone | (Inherited from Object.) |
|
ToString | (Inherited from Object.) |
Top
Remarks
When your database provider retrieves table data using the GetData method, your provider must specify the column metadata for your table's data before the database manager can query the information.
Examples
The following code sample implements the GetData method that is used to retrieve the data in a database table from an OLEDB data source.
Note
This example also provides helper methods that perform additional database tasks.
' Retrieve data from a database.
Public Function GetData( _
ByVal connectionString As String, _
ByVal tableName As String, _
ByVal schema As String) _
As Microsoft.Web.Management.DatabaseManager.QueryResult _
Implements Microsoft.Web.Management.DatabaseManager.IDbTableDataManager.GetData
' Create a new query result object.
Dim queryResult As QueryResult = New QueryResult
' Create a connection to the database.
Dim connection As OleDbConnection = New OleDbConnection(connectionString)
' Open the database connection.
connection.Open()
Dim columns As List(Of Column) = New List(Of Column)
GetColumns(connection, tableName, schema, columns)
Dim metadata As IList(Of QueryColumnMetadata) = New List(Of QueryColumnMetadata)
For Each c As Column In columns
Dim m As QueryColumnMetadata = New QueryColumnMetadata
m.Name = c.Name
m.ColumnWidth = c.Length
m.IsNullable = c.AllowNulls
m.TypeInfo = c.TypeInfo
metadata.Add(m)
Next
Dim fields As StringBuilder = New StringBuilder
If (metadata.Count = 0) Then
' When the table is not found, adding an asterix returns an error message.
fields.Append("*")
Else
Dim fieldCount As Integer = metadata.Count
Dim isFirst As Boolean = True
Dim i As Integer = 0
For i = 0 To (fieldCount - 1)
Dim m As QueryColumnMetadata = metadata(i)
If (isFirst = False) Then
fields.Append(",")
End If
isFirst = False
fields.Append(EscapeName(m.Name))
queryResult.ColumnMetadata.Add(m)
Next
End If
' Create a new query object.
Dim query As Query = New Query
' Create a SQL query statement.
query.Statement = "SELECT " + fields.ToString + " FROM " + EscapeName(tableName)
' retrieve the query results.
Dim results() As QueryResult = ExecuteQuery(connectionString, query)
' Add the query results to the list.
queryResult.QueryResults.AddRange(results(0).QueryResults)
' Return the query result.
Return queryResult
End Function
...
Friend Shared Function EscapeName(ByVal name As String) As String
' Test if the value is already escaped.
If (name.StartsWith("[", StringComparison.Ordinal) AndAlso name.EndsWith("]", StringComparison.Ordinal)) Then
' Return the string with no changes.
Return name
End If
' Create a new string builder for the escaped string.
Dim escapedName As StringBuilder = New StringBuilder
' Prepend the string with an opening bracket.
escapedName.Append(Microsoft.VisualBasic.ChrW(91))
' Loop through the string.
Dim i As Integer = 0
Do While (i < name.Length)
Dim c As Char = name(i)
If (c = Microsoft.VisualBasic.ChrW(93)) Then
escapedName.Append(Microsoft.VisualBasic.ChrW(93))
End If
escapedName.Append(c)
i = (i + 1)
Loop
' Append the string with an opening bracket.
escapedName.Append(Microsoft.VisualBasic.ChrW(93))
' Return the escaped string.
Return escapedName.ToString
End Function
// Retrieve data from a database.
public QueryResult GetData(string connectionString, string tableName, string schema)
{
// Create a new query result object.
QueryResult queryResult = new QueryResult();
// Create a connection to the database.
using (OleDbConnection connection = new OleDbConnection(connectionString))
{
// Open the database connection.
connection.Open();
List<Column> columns = new List<Column>();
GetColumns(connection, tableName, schema, columns);
IList<QueryColumnMetadata> metadata = new List<QueryColumnMetadata>();
foreach (Column c in columns)
{
QueryColumnMetadata m = new QueryColumnMetadata();
m.Name = c.Name;
m.ColumnWidth = c.Length;
m.IsNullable = c.AllowNulls;
m.TypeInfo = c.TypeInfo;
metadata.Add(m);
}
StringBuilder fields = new StringBuilder();
if (metadata.Count == 0)
{
// When the table is not found, adding an asterix returns an error message.
fields.Append("*");
}
else
{
int fieldCount = metadata.Count;
bool isFirst = true;
for (int i = 0; i < fieldCount; i++)
{
QueryColumnMetadata m = metadata[i];
if (isFirst == false)
{
fields.Append(',');
}
isFirst = false;
fields.Append(EscapeName(m.Name));
queryResult.ColumnMetadata.Add(m);
}
}
// Create a new query object.
Query query = new Query();
// Create a SQL query statement.
query.Statement = "SELECT " + fields.ToString() + " FROM " + EscapeName(tableName);
// retrieve the query results.
QueryResult[] results = ExecuteQuery(connectionString, query);
// Add the query results to the list.
queryResult.QueryResults.AddRange(results[0].QueryResults);
}
// Return the query result.
return queryResult;
}
...
internal static string EscapeName(string name)
{
// Test if the value is already escaped.
if (name.StartsWith("[", StringComparison.Ordinal) && name.EndsWith("]", StringComparison.Ordinal))
{
// Return the string with no changes.
return name;
}
// Create a new string builder for the escaped string.
StringBuilder escapedName = new StringBuilder();
// Prepend the string with an opening bracket.
escapedName.Append('[');
// Loop through the string.
for (int i = 0; i < name.Length; i++)
{
char c = name[i];
if (c == ']')
{
escapedName.Append(']');
}
escapedName.Append(c);
}
// Append the string with an opening bracket.
escapedName.Append(']');
// Return the escaped string.
return escapedName.ToString();
}
Thread Safety
Any public static (Shared in Visual Basic) members of this type are thread safe. Any instance members are not guaranteed to be thread safe.