Public Sub BeginFlush() 

public void BeginFlush()

using (ManualResetEvent mre = new ManualResetEvent(false))
using (TempFile temp = new TempFile())
using (TestStream io = new TestStream(temp.Open()))
using (BackgroundWriter wtr = new BackgroundWriter(io))
{
    const int sz = 1000;
    const int iter = 10000;
    var bytes = new byte[sz];
    for (int i = 0; i < iter; i++)
        wtr.Write(bytes, 0, sz);
    wtr.BeginFlush();
    Assert.IsFalse(io.Flushed);

    wtr.Perform(s => mre.Set());
    Assert.IsTrue(mre.WaitOne(60000, false));

    Assert.IsTrue(io.Flushed);
    Assert.AreEqual(sz * iter, io.Position);
    Assert.AreEqual(sz * iter, io.Length);
}

Using mre As New ManualResetEvent(False)
    Using temp As New TempFile()
        Using io As New TestStream(temp.Open())
            Using wtr As New BackgroundWriter(io)
                Const  sz As Integer = 1000
                Const  iter As Integer = 10000
                Dim bytes As var = New Byte(sz) {}
                Dim i As Integer = 0
                While i < iter
                    wtr.Write(bytes, 0, sz)
                    System.Math.Max(System.Threading.Interlocked.Increment(i),i - 1)
                End While
                wtr.BeginFlush()
                Assert.IsFalse(io.Flushed)

                wtr.Perform(Function(s) mre.[Set]())
                Assert.IsTrue(mre.WaitOne(60000, False))

                Assert.IsTrue(io.Flushed)
                Assert.AreEqual(sz * iter, io.Position)
                Assert.AreEqual(sz * iter, io.Length)
            End Using
        End Using
    End Using
End Using