StreamReader отличие методов чтения Read и ReadBlock

// Blocking version of read.  Returns only when count
// characters have been read or the end of the file was reached.
//
public virtual int ReadBlock(char[] buffer, int index, int count)
{
    int i, n = 0;
    do
    {
        n += (i = Read(buffer, index + n, count - n));
    } while (i > 0 && n < count);
    return n;
}

using var r = new StreamReader(new BaseStream());
var buff = new char[16];
void Test(Func<char[], int, int, int> method)
{ 
    var sw = Stopwatch.StartNew();
    var count = method(buff, 0, buff.Length);
    sw.Stop();
    Console.WriteLine($"time: {sw.Elapsed}; count = {count}; result = {new string(buff)}");
}
Test(r.Read);
Test(r.ReadBlock);
;
 
class BaseStream : Stream
{
    private readonly byte[] _array = Encoding.ASCII.GetBytes("Nya!!");
    public override int Read(byte[] buffer, int offset, int count)
    {
        Thread.Sleep(TimeSpan.FromSeconds(1));
        var c = Math.Min(_array.Length, count);
        Array.Copy(_array, 0, buffer, offset, c);
        return c;
    }
 
    public override bool CanRead => true;
    public override bool CanSeek => false;
    public override bool CanWrite => false;
    public override long Length => throw new NotSupportedException();
    public override long Position { get => throw new NotSupportedException(); set => throw new NotSupportedException(); }
 
    public override void Flush()
        => throw new NotSupportedException();
    public override long Seek(long offset, SeekOrigin origin)
        => throw new NotSupportedException();
    public override void SetLength(long value)
        => throw new NotSupportedException();
    public override void Write(byte[] buffer, int offset, int count)
        => throw new NotSupportedException();
}

 // Reads a block of characters. This method will read up to
 // count characters from this TextReader into the
 // buffer character array starting at position
 // index. Returns the actual number of characters read.
 //
 public virtual int Read(char[] buffer, int index, int count)
 {
     ArgumentNullException.ThrowIfNull(buffer);
 
     ArgumentOutOfRangeException.ThrowIfNegative(index);
     ArgumentOutOfRangeException.ThrowIfNegative(count);
     if (buffer.Length - index < count)
     {
         throw new ArgumentException(SR.Argument_InvalidOffLen);
     }
 
     int n;
     for (n = 0; n < count; n++)
     {
         int ch = Read();
         if (ch == -1) break;
         buffer[index + n] = (char)ch;
     }
 
     return n;
 }

// Reads the next character from the input stream. The returned value is
// -1 if no further characters are available.
//
// This default method simply returns -1.
//
public virtual int Read()
{
    return -1;
}