StreamReader.ReadLineAsync Method

Definition

Namespace:

System.IO

Assembly:

System.Runtime.dll

Overloads

ReadLineAsync()	Reads a line of characters asynchronously from the current stream and returns the data as a string.
ReadLineAsync(CancellationToken)	Reads a line of characters asynchronously from the current stream and returns the data as a string.

ReadLineAsync()

Source:

StreamReader.cs

Reads a line of characters asynchronously from the current stream and returns the data as a string.

public:
 override System::Threading::Tasks::Task<System::String ^> ^ ReadLineAsync();

public override System.Threading.Tasks.Task<string?> ReadLineAsync ();

override this.ReadLineAsync : unit -> System.Threading.Tasks.Task<string>

Public Overrides Function ReadLineAsync () As Task(Of String)

Returns

Task<String>

A task that represents the asynchronous read operation. The value of the TResult parameter contains the next line from the stream, or is null if all the characters have been read.

Exceptions

ArgumentOutOfRangeException

The number of characters in the next line is larger than Int32.MaxValue.

ObjectDisposedException

The stream has been disposed.

InvalidOperationException

The reader is currently in use by a previous read operation.

Examples

The following example shows how to read the first line of a file by using the ReadLineAsync() method.

using System;
using System.IO;

namespace ConsoleApplication
{
    class Program
    {
        static async Task Main()
        {
            await ReadCharacters();
        }

        static async Task ReadCharacters()
        {
            String result;
            using (StreamReader reader = File.OpenText("existingfile.txt"))
            {
                Console.WriteLine("Opened file.");
                result = await reader.ReadLineAsync();
                Console.WriteLine("First line contains: " + result);
            }
        }
    }
}

Imports System.IO

Module Module1

    Sub Main()
        ReadCharacters()
    End Sub

    Async Sub ReadCharacters()
        Dim result As String

        Using reader As StreamReader = File.OpenText("existingfile.txt")
            Console.WriteLine("Opened file.")
            result = Await reader.ReadLineAsync()
            Console.WriteLine("First line contains: " + result)
        End Using
    End Sub
End Module

Remarks

This method stores in the task it returns all non-usage exceptions that the method's synchronous counterpart can throw. If an exception is stored into the returned task, that exception will be thrown when the task is awaited. Usage exceptions, such as ArgumentException, are still thrown synchronously. For the stored exceptions, see the exceptions thrown by ReadLine().

ReadLineAsync(CancellationToken)

Source:

StreamReader.cs

Reads a line of characters asynchronously from the current stream and returns the data as a string.

public:
 override System::Threading::Tasks::ValueTask<System::String ^> ReadLineAsync(System::Threading::CancellationToken cancellationToken);

public override System.Threading.Tasks.ValueTask<string?> ReadLineAsync (System.Threading.CancellationToken cancellationToken);

override this.ReadLineAsync : System.Threading.CancellationToken -> System.Threading.Tasks.ValueTask<string>

Public Overrides Function ReadLineAsync (cancellationToken As CancellationToken) As ValueTask(Of String)

Parameters

cancellationToken

CancellationToken

The token to monitor for cancellation requests.

Returns

ValueTask<String>

A value task that represents the asynchronous read operation. The value of the TResult parameter contains the next line from the stream, or is null if all of the characters have been read.

Exceptions

ArgumentOutOfRangeException

The number of characters in the next line is larger than Int32.MaxValue.

ObjectDisposedException

The stream reader has been disposed.

InvalidOperationException

The reader is currently in use by a previous read operation.

OperationCanceledException

The cancellation token was canceled. This exception is stored into the returned task.

Remarks

If this method is canceled via cancellationToken, some data that has been read from the current Stream but not stored (by the StreamReader) or returned (to the caller) may be lost.

This method stores in the task it returns all non-usage exceptions that the method's synchronous counterpart can throw. If an exception is stored into the returned task, that exception will be thrown when the task is awaited. Usage exceptions, such as ArgumentException, are still thrown synchronously. For the stored exceptions, see the exceptions thrown by ReadLine().