有趣的問題。把它帶到反編譯源,當您編譯查詢,這是發生了什麼:
public static Func<TArg0, TArg1, TResult> Compile<TArg0, TArg1, TResult>(Expression<Func<TArg0, TArg1, TResult>> query) where TArg0 : DataContext
{
if (query == null)
System.Data.Linq.Error.ArgumentNull("query");
if (CompiledQuery.UseExpressionCompile((LambdaExpression) query))
return query.Compile();
else
return new Func<TArg0, TArg1, TResult>(new CompiledQuery((LambdaExpression) query).Invoke<TArg0, TArg1, TResult>);
}
的UseExpressionCompile方法的定義如下:
private static bool UseExpressionCompile(LambdaExpression query)
{
return typeof (ITable).IsAssignableFrom(query.Body.Type);
}
這個計算結果爲假的表達式您已經定義,所以使用else情況。
的調用是這樣的:
private TResult Invoke<TArg0, TArg1, TResult>(TArg0 arg0, TArg1 arg1) where TArg0 : DataContext
{
return (TResult) this.ExecuteQuery((DataContext) arg0, new object[2]
{
(object) arg0,
(object) arg1
});
}
的的executeQuery是這樣的:
private object ExecuteQuery(DataContext context, object[] args)
{
if (context == null)
throw System.Data.Linq.Error.ArgumentNull("context");
if (this.compiled == null)
{
lock (this)
{
if (this.compiled == null)
this.compiled = context.Provider.Compile((Expression) this.query);
}
}
return this.compiled.Execute(context.Provider, args).ReturnValue;
}
在這種情況下,我們的供應商是sqlProvider的課,SqlProvider.CompiledQuery是實現ICompiledQuery類。在該類執行實現:
public IExecuteResult Execute(IProvider provider, object[] arguments)
{
if (provider == null)
throw System.Data.Linq.SqlClient.Error.ArgumentNull("provider");
SqlProvider sqlProvider = provider as SqlProvider;
if (sqlProvider == null)
throw System.Data.Linq.SqlClient.Error.ArgumentTypeMismatch((object) "provider");
if (!SqlProvider.CompiledQuery.AreEquivalentShapes(this.originalShape, sqlProvider.services.Context.LoadOptions))
throw System.Data.Linq.SqlClient.Error.CompiledQueryAgainstMultipleShapesNotSupported();
else
return sqlProvider.ExecuteAll(this.query, this.queryInfos, this.factory, arguments, this.subQueries);
}
SqlProvider.ExecuteAll調用SqlProvider.Execute,這是一個相當大的方法,所以我會發布的亮點:
private IExecuteResult Execute(Expression query, SqlProvider.QueryInfo queryInfo, IObjectReaderFactory factory, object[] parentArgs, object[] userArgs, ICompiledSubQuery[] subQueries, object lastResult)
{
this.InitializeProviderMode();
DbConnection dbConnection = this.conManager.UseConnection((IConnectionUser) this);
try
{
DbCommand command = dbConnection.CreateCommand();
command.CommandText = queryInfo.CommandText;
command.Transaction = this.conManager.Transaction;
command.CommandTimeout = this.commandTimeout;
this.AssignParameters(command, queryInfo.Parameters, userArgs, lastResult);
this.LogCommand(this.log, command);
++this.queryCount;
switch (queryInfo.ResultShape)
{
case SqlProvider.ResultShape.Singleton:
DbDataReader reader1 = command.ExecuteReader();
...
case SqlProvider.ResultShape.Sequence:
DbDataReader reader2 = command.ExecuteReader();
...
default:
return (IExecuteResult) new SqlProvider.ExecuteResult(command, queryInfo.Parameters, (IObjectReaderSession) null, (object) command.ExecuteNonQuery(), true);
}
}
finally
{
this.conManager.ReleaseConnection((IConnectionUser) this);
}
}
在獲取和釋放之間連接它優於SQL命令。所以我會說你是對的。與流行的觀點相反,編譯查詢在延遲執行時的行爲與未編譯查詢的行爲不同。
我敢肯定,你可以從MS下載實際的源代碼,但我沒有它方便,Resharper 6有一個很棒的去反編譯函數,所以我只是用它。
你說得對。編譯後的查詢不可組合,它們在調用委託而不是查詢本身時返回結果。我改變了我對所引用問題的回答。 – alex