TextReader.ReadAsync Method

Definition

Namespace:

System.IO

Assembly:

System.Runtime.dll

Overloads

ReadAsync(Memory<Char>, CancellationToken)	Asynchronously reads the characters from the current stream into a memory block.
ReadAsync(Char[], Int32, Int32)	Reads a specified maximum number of characters from the current text reader asynchronously and writes the data to a buffer, beginning at the specified index.

ReadAsync(Memory<Char>, CancellationToken)

Source:

TextReader.cs

Asynchronously reads the characters from the current stream into a memory block.

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

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

Public Overridable 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 specified memory block of characters replaced by 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 value task that represents the asynchronous read operation. The value of the type parameter contains the number of characters that have been read, or 0 if at the end of the stream and no data was read. The number will be less than or equal to the buffer length, depending on whether the data is available within the stream.

Exceptions

OperationCanceledException

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

ReadAsync(Char[], Int32, Int32)

Source:

TextReader.cs

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

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

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

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

Public Overridable 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 text is reached before the specified number of characters is read into the buffer, the current 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 text 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 text reader has been disposed.

InvalidOperationException

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

Remarks

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

The TextReader class is an abstract class. Therefore, you do not instantiate it in your code. For an example of using the ReadAsync method, see the StreamReader.ReadAsync method.

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).

See also

• ReadAsync(Char[], Int32, Int32)
• ReadAsync(Char[], Int32, Int32)
• File and Stream I/O
• How to: Read Text from a File
• How to: Write Text to a File