SqlCommand.BeginExecuteReader Method (AsyncCallback, Object, CommandBehavior)


The .NET API Reference documentation has a new home. Visit the .NET API Browser on docs.microsoft.com to see the new experience.

Initiates the asynchronous execution of the Transact-SQL statement or stored procedure that is described by this SqlCommand, using one of the CommandBehavior values, and retrieving one or more result sets from the server, given a callback procedure and state information.

Namespace:   System.Data.SqlClient
Assembly:  System.Data (in System.Data.dll)

[HostProtectionAttribute(SecurityAction.LinkDemand, ExternalThreading = true)]
public IAsyncResult BeginExecuteReader(
	AsyncCallback callback,
	object stateObject,
	CommandBehavior behavior


Type: System.AsyncCallback

An AsyncCallback delegate that is invoked when the command's execution has completed. Pass null (Nothing in Microsoft Visual Basic) to indicate that no callback is required.

Type: System.Object

A user-defined state object that is passed to the callback procedure. Retrieve this object from within the callback procedure using the AsyncState property.

Type: System.Data.CommandBehavior

One of the CommandBehavior values, indicating options for statement execution and data retrieval.

Return Value

Type: System.IAsyncResult

An IAsyncResult that can be used to poll or wait for results, or both; this value is also needed when invoking EndExecuteReader, which returns a SqlDataReader instance which can be used to retrieve the returned rows.

Exception Condition

A SqlDbType other than Binary or VarBinary was used when Value was set to Stream. For more information about streaming, see SqlClient Streaming Support.

A SqlDbType other than Char, NChar, NVarChar, VarChar, or Xml was used when Value was set to TextReader.

A SqlDbType other than Xml was used when Value was set to XmlReader.


Any error that occurred while executing the command text.

A timeout occurred during a streaming operation. For more information about streaming, see SqlClient Streaming Support.


The name/value pair "Asynchronous Processing=true" was not included within the connection string defining the connection for this SqlCommand.

The SqlConnection closed or dropped during a streaming operation. For more information about streaming, see SqlClient Streaming Support.


An error occurred in a Stream, XmlReader or TextReader object during a streaming operation. For more information about streaming, see SqlClient Streaming Support.


The Stream, XmlReader or TextReader object was closed during a streaming operation. For more information about streaming, see SqlClient Streaming Support.

The BeginExecuteReader method starts the process of asynchronously executing a Transact-SQL statement or stored procedure that returns rows, so that other tasks can run concurrently while the statement is executing. When the statement has completed, developers must call the EndExecuteReader method to finish the operation and retrieve the SqlDataReader returned by the command. The BeginExecuteReader method returns immediately, but until the code executes the corresponding EndExecuteReader method call, it must not execute any other calls that start a synchronous or asynchronous execution against the same SqlCommand object. Calling the EndExecuteReader before the command's execution is completed causes the SqlCommand object to block until the execution is finished.

The callback parameter lets you specify an AsyncCallback delegate that is called when the statement has completed. You can call the EndExecuteReader method from within this delegate procedure, or from any other location within your application. In addition, you can pass any object in the stateObject parameter, and your callback procedure can retrieve this information using the AsyncState property.

The behavior parameter lets you specify options that control the behavior of the command and its connection. These values can be combined together (using the programming language's Or operator); generally, developers use the CloseConnection value to make sure that the connection is closed by the runtime when the SqlDataReader is closed. Developers can also optimize the behavior of the SqlDataReader by specifying the SingleRow value when it is known in advance that the Transact-SQL statement or stored procedure only returns a single row.

Note that the command text and parameters are sent to the server synchronously. If a large command or many parameters are sent, this method may block during writes. After the command is sent, the method returns immediately without waiting for an answer from the server--that is, reads are asynchronous. Although command execution is asynchronous, value fetching is still synchronous. This means that calls to Read may block if more data is required and the underlying network's read operation blocks.

Because the callback procedure executes from within a background thread supplied by the Microsoft .NET common language runtime, it is very important that you take a rigorous approach to handling cross-thread interactions from within your applications. For example, you must not interact with a form's contents from within your callback procedure--should you have to update the form, you must switch back to the form's thread in order to do your work. The example in this topic demonstrates this behavior.

All errors that occur during the execution of the operation are thrown as exceptions in the callback procedure. You must handle the exception in the callback procedure, not in the main application. See the example in this topic for additional information on handling exceptions in the callback procedure.

If you use ExecuteReader or BeginExecuteReader to access XML data, SQL Server will return any XML results greater than 2,033 characters in length in multiple rows of 2,033 characters each. To avoid this behavior, use ExecuteXmlReader or BeginExecuteXmlReader to read FOR XML queries. For more information, see article Q310378, "PRB: XML Data Is Truncated When You Use SqlDataReader," in the Microsoft Knowledge Base at http://support.microsoft.com.

The following Windows application demonstrates the use of the BeginExecuteReader method, executing a Transact-SQL statement that includes a delay of a few seconds (emulating a long-running command). Because the sample executes the command asynchronously, the form remains responsive while awaiting the results. This example passes the executing SqlCommand object as the stateObject parameter; doing so makes it simple to retrieve the SqlCommand object from within the callback procedure, so that the code can call the EndExecuteReader method corresponding to the initial call to BeginExecuteReader.

This example demonstrates many important techniques. This includes calling a method that interacts with the form from a separate thread. In addition, this example demonstrates how you must block users from executing a command multiple times concurrently, and how you must make sure that the form does not close before the callback procedure is called.

To set up this example, create a new Windows application. Put a Button control, a DataGridView control, and a Label control on the form (accepting the default name for each control). Add the following code to the form's class, modifying the connection string as needed for your environment.

This example passes the CommandBehavior.CloseConnection value in the behavior parameter, causing the returned SqlDataReader to automatically close its connection when it is closed.

using System.Data.SqlClient;

namespace Microsoft.AdoDotNet.CodeSamples
    public partial class Form1 : Form
        public Form1()
        // Hook up the form's Load event handler (you can double-click on 
        // the form's design surface in Visual Studio), and then add 
        // this code to the form's class:
        // You need this delegate in order to fill the grid from
        // a thread other than the form's thread. See the HandleCallback
        // procedure for more information.
        private delegate void FillGridDelegate(SqlDataReader reader);

        // You need this delegate to update the status bar.
        private delegate void DisplayStatusDelegate(string Text);

        // This flag ensures that the user does not attempt
        // to restart the command or close the form while the 
        // asynchronous command is executing.
        private bool isExecuting;

        private void DisplayStatus(string Text)
            this.label1.Text = Text;

        private void FillGrid(SqlDataReader reader)
                DataTable table = new DataTable();
                this.dataGridView1.DataSource = table;
            catch (Exception ex)
                // Because you are guaranteed this procedure
                // is running from within the form's thread,
                // it can directly interact with members of the form.
                DisplayStatus(string.Format("Ready (last attempt failed: {0})",
                // Closing the reader also closes the connection,
                // because this reader was created using the 
                // CommandBehavior.CloseConnection value.
                if (reader != null)

        private void HandleCallback(IAsyncResult result)
                // Retrieve the original command object, passed
                // to this procedure in the AsyncState property
                // of the IAsyncResult parameter.
                SqlCommand command = (SqlCommand)result.AsyncState;
                SqlDataReader reader = command.EndExecuteReader(result);
                // You may not interact with the form and its contents
                // from a different thread, and this callback procedure
                // is all but guaranteed to be running from a different thread
                // than the form. Therefore you cannot simply call code that 
                // fills the grid, like this:
                // FillGrid(reader);
                // Instead, you must call the procedure from the form's thread.
                // One simple way to accomplish this is to call the Invoke
                // method of the form, which calls the delegate you supply
                // from the form's thread. 
                FillGridDelegate del = new FillGridDelegate(FillGrid);
                this.Invoke(del, reader);
                // Do not close the reader here, because it is being used in 
                // a separate thread. Instead, have the procedure you have
                // called close the reader once it is done with it.
            catch (Exception ex)
                // Because you are now running code in a separate thread, 
                // if you do not handle the exception here, none of your other
                // code catches the exception. Because there is none of 
                // your code on the call stack in this thread, there is nothing
                // higher up the stack to catch the exception if you do not 
                // handle it here. You can either log the exception or 
                // invoke a delegate (as in the non-error case in this 
                // example) to display the error on the form. In no case
                // can you simply display the error without executing a delegate
                // as in the try block here. 
                // You can create the delegate instance as you 
                // invoke it, like this:
                this.Invoke(new DisplayStatusDelegate(DisplayStatus), "Error: " +
                isExecuting = false;

        private string GetConnectionString()
            // To avoid storing the connection string in your code, 
            // you can retrieve it from a configuration file. 

            // If you do not include the Asynchronous Processing=true name/value pair,
            // you wo not be able to execute the command asynchronously.
            return "Data Source=(local);Integrated Security=true;" +
                "Initial Catalog=AdventureWorks; Asynchronous Processing=true";

        private void button1_Click(object sender, System.EventArgs e)
            if (isExecuting)
                    "Already executing. Please wait until the current query " +
                    "has completed.");
                SqlCommand command = null;
                SqlConnection connection = null;
                    connection = new SqlConnection(GetConnectionString());
                    // To emulate a long-running query, wait for 
                    // a few seconds before retrieving the real data.
                    command = new SqlCommand("WAITFOR DELAY '0:0:5';" +
                        "SELECT ProductID, Name, ListPrice, Weight FROM Production.Product",

                    isExecuting = true;
                    // Although it is not required that you pass the 
                    // SqlCommand object as the second parameter in the 
                    // BeginExecuteReader call, doing so makes it easier
                    // to call EndExecuteReader in the callback procedure.
                    AsyncCallback callback = new AsyncCallback(HandleCallback);
                    command.BeginExecuteReader(callback, command,
                catch (Exception ex)
                    DisplayStatus("Error: " + ex.Message);
                    if (connection != null)

        private void Form1_Load(object sender, System.EventArgs e)
            this.button1.Click += new System.EventHandler(this.button1_Click);
            this.FormClosing += new FormClosingEventHandler(Form1_FormClosing);

        void Form1_FormClosing(object sender, FormClosingEventArgs e)
            if (isExecuting)
                MessageBox.Show(this, "Cannot close the form until " +
                    "the pending asynchronous command has completed. Please wait...");
                e.Cancel = true;

.NET Framework
Available since 2.0
Return to top