StreamReader.ReadLineAsync() returns string.empty in release mode but not in debug mode (in Blazor WASM app)

Question

Using Blazor wasm app (NET standard 2.1) and the BlazorFileReader in the following code snippet (shortened, in my app it is divided into a .razor file and a .razor.cs file but shortened and put together here for the sake of simplicity):

@inject Blazor.FileReader.IFileReaderService fileReaderService
@using Blazor.FileReader;
    <div class="ml-2">
        <input type="file" @ref="InputElement" @onchange="FilesSelected" multiple accept=".txt,.csv" />
    </div>

@code {
        ElementReference InputElement;
        [Parameter] public List<string> FileContent { get; set; }
        async Task FilesSelected()
        {
            foreach (var file in await fileReaderService.CreateReference(InputElement).EnumerateFilesAsync())
            {
                using (Stream stream = await file.OpenReadAsync())
                {
                    FileContent = await ReadLinesAsync(stream, Encoding.UTF8);
                }
            }
        }
        public async Task<List<string>> ReadLinesAsync(Stream stream, Encoding encoding)
        {
            using (var reader = new StreamReader(stream, encoding))
            {
                string line;
                var result = new List<string>();
                while ((line = await reader.ReadLineAsync()) != null)
                {
                    result.Add(line);
                }
                return result;
            }
        }
}

Running this code in debug mode (Visual studio 2019 v 16.6 preview) reads any text file just fine. But when running in release mode the StreamReader.ReadLineAsync() returns empty strings.

Does anyone know why ReadLineAsync() does this? I have looked at this...

Tewr · Accepted Answer · 2020-06-05T18:46:01.293

This is a bug in the current implementation of Blazor.

Until it is resolved, here is a workaround (the code is more or less equivalent... but works!):

    public async Task<List<string>> ReadLinesAsync(Stream stream, Encoding encoding)
    {
        using (var reader = new StreamReader(stream, encoding))
        {

            var result = new List<string>();
            string line = await reader.ReadLineAsync();
            while (line != null)
            {
                result.Add(line);
                line = await reader.ReadLineAsync();
            }
            return result;
        }
    }

Edit: The bug seems to be in the Optimizer. Turning off "Optimize" (on by default when compiling in Release) is an alternative workaround.

StreamReader.ReadLineAsync() returns string.empty in release mode but not in debug mode (in Blazor WASM app)

1 Answers1