StringReader.ReadAsync Method

Definition

Namespace:

System.IO

Assembly:

System.Runtime.dll

Overloads

ReadAsync(Char[], Int32, Int32)	Reads a specified maximum number of characters from the current string asynchronously and writes the data to a buffer, beginning at the specified index.
ReadAsync(Memory<Char>, CancellationToken)	Asynchronously reads all the characters from the input string, starting at the current position, and advances the current position to the end of the input string.

ReadAsync(Char[], Int32, Int32)

Source:

StringReader.cs

Reads a specified maximum number of characters from the current string asynchronously and writes the data to a buffer, beginning at the specified index.

public:
 override System::Threading::Tasks::Task<int> ^ ReadAsync(cli::array <char> ^ buffer, int index, int count);

public override System.Threading.Tasks.Task<int> ReadAsync (char[] buffer, int index, int count);

override this.ReadAsync : char[] * int * int -> System.Threading.Tasks.Task<int>

Public Overrides Function ReadAsync (buffer As Char(), index As Integer, count As Integer) As Task(Of Integer)

Parameters

buffer

Char[]

When this method returns, contains the specified character array with the values between index and (index + count - 1) replaced by the characters read from the current source.

index

Int32

The position in buffer at which to begin writing.

count

Int32

The maximum number of characters to read. If the end of the string is reached before the specified number of characters is written into the buffer, the method returns.

Returns

Task<Int32>

A task that represents the asynchronous read operation. The value of the TResult parameter contains the total number of bytes read into the buffer. The result value can be less than the number of bytes requested if the number of bytes currently available is less than the requested number, or it can be 0 (zero) if the end of the string has been reached.

Exceptions

ArgumentNullException

buffer is null.

ArgumentOutOfRangeException

index or count is negative.

ArgumentException

The sum of index and count is larger than the buffer length.

ObjectDisposedException

The string reader 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 23 characters of a string asynchronously.

using System;
using System.IO;

namespace ConsoleApplication
{
    class Program
    {
        static void Main(string[] args)
        {
            ReadCharacters();
        }

        static async void ReadCharacters()
        {
            string stringToRead = "Some characters to read but not all";
            char[] charsRead = new char[stringToRead.Length];

            using (StringReader reader = new StringReader(stringToRead))
            {
                await reader.ReadAsync(charsRead, 0, 23);
                Console.WriteLine(charsRead);
            }
        }
    }
}
// The example displays the following output:
// Some characters to read
//

Imports System.IO

Module Module1

    Sub Main()
        ReadCharacters()
    End Sub

    Async Sub ReadCharacters()
        Dim stringToRead = "Some characters to read but not all"
        Dim charsRead(stringToRead.Length) As Char

        Using reader As StringReader = New StringReader(stringToRead)
            Await reader.ReadAsync(charsRead, 0, 23)
            Console.WriteLine(charsRead)
        End Using
    End Sub

End Module
' The example displays the following output:
' Some characters to read
'

Remarks

The task completes after either the number of characters specified by the count parameter are read or the end of the string is reached.

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 Read(Char[], Int32, Int32).

ReadAsync(Memory<Char>, CancellationToken)

Source:

StringReader.cs

Asynchronously reads all the characters from the input string, starting at the current position, and advances the current position to the end of the input string.

public override System.Threading.Tasks.ValueTask<int> ReadAsync (Memory<char> buffer, System.Threading.CancellationToken cancellationToken = default);

override this.ReadAsync : Memory<char> * System.Threading.CancellationToken -> System.Threading.Tasks.ValueTask<int>

Public Overrides Function ReadAsync (buffer As Memory(Of Char), Optional cancellationToken As CancellationToken = Nothing) As ValueTask(Of Integer)

Parameters

buffer

Memory<Char>

When this method returns, contains the characters read from the current source.

cancellationToken

CancellationToken

The token to monitor for cancellation requests. The default value is None.

Returns

ValueTask<Int32>

A task that represents the asynchronous read operation. The value of the TResult parameter contains the total number of characters read into the buffer.

Exceptions

OperationCanceledException

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