c# - Mock File.OpenRead - Stack Overflow

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I am trying to use MoQ and System.IO.Abstraction to mock File.OpenRead function..

Sample:

using System;
using System.IO;
using System.IO.Abstractions;
using Microsoft.VisualStudio.TestTools.UnitTesting;
using Moq;

[TestClass]
public class FileServiceTests
{
    [TestMethod]
    public void OpenRead_ShouldThrowIOException_ThenReturnValidStream()
    {
        // Arrange
        var filePath = "testfile.txt";
        var expectedContent = "Hello, World!";
        var fileBytes = System.Text.Encoding.UTF8.GetBytes(expectedContent);
        var memoryStream = new MemoryStream(fileBytes);

        var mockFileSystem = new Mock<IFileSystem>();
        var mockFile = new Mock<IFile>();

        mockFileSystem.Setup(fs => fs.File).Returns(mockFile.Object);

        mockFile.Setup(f => f.OpenRead(filePath))
                .Returns(memoryStream);

        var fileService = new FileService(mockFileSystem.Object);

        using var resultStream = fileService.OpenFile(filePath);
        using var reader = new StreamReader(resultStream);
        var resultContent = reader.ReadToEnd();

        // Assert
        Assert.AreEqual(expectedContent, resultContent);
    }
}

// Example service that uses IFileSystem for OpenRead
public class FileService
{
    private readonly IFileSystem _fileSystem;

    public FileService(IFileSystem fileSystem)
    {
        _fileSystem = fileSystem;
    }

    public Stream OpenFile(string path)
    {
        return _fileSystem.File.OpenRead(path);
    }
}

Issue:

mockFile.Setup(f => f.OpenRead(filePath)) .Returns(memoryStream);

Here I am trying to pass instance of mocked FileStream or MemoryStream.. But, I get an error when I start the test.. The OpenRead expects FileSystemStream here in "Returns".

Not sure how I can solve this..

I am trying to use MoQ and System.IO.Abstraction to mock File.OpenRead function..

Sample:

using System;
using System.IO;
using System.IO.Abstractions;
using Microsoft.VisualStudio.TestTools.UnitTesting;
using Moq;

[TestClass]
public class FileServiceTests
{
    [TestMethod]
    public void OpenRead_ShouldThrowIOException_ThenReturnValidStream()
    {
        // Arrange
        var filePath = "testfile.txt";
        var expectedContent = "Hello, World!";
        var fileBytes = System.Text.Encoding.UTF8.GetBytes(expectedContent);
        var memoryStream = new MemoryStream(fileBytes);

        var mockFileSystem = new Mock<IFileSystem>();
        var mockFile = new Mock<IFile>();

        mockFileSystem.Setup(fs => fs.File).Returns(mockFile.Object);

        mockFile.Setup(f => f.OpenRead(filePath))
                .Returns(memoryStream);

        var fileService = new FileService(mockFileSystem.Object);

        using var resultStream = fileService.OpenFile(filePath);
        using var reader = new StreamReader(resultStream);
        var resultContent = reader.ReadToEnd();

        // Assert
        Assert.AreEqual(expectedContent, resultContent);
    }
}

// Example service that uses IFileSystem for OpenRead
public class FileService
{
    private readonly IFileSystem _fileSystem;

    public FileService(IFileSystem fileSystem)
    {
        _fileSystem = fileSystem;
    }

    public Stream OpenFile(string path)
    {
        return _fileSystem.File.OpenRead(path);
    }
}

Issue:

mockFile.Setup(f => f.OpenRead(filePath)) .Returns(memoryStream);

Here I am trying to pass instance of mocked FileStream or MemoryStream.. But, I get an error when I start the test.. The OpenRead expects FileSystemStream here in "Returns".

Not sure how I can solve this..

• c#
• unit-testing
• system.io.file

Share Follow asked Mar 7 at 13:52 njdotnetdevnjdotnetdev 1566 bronze badges 1

• I haven't used System.IO.Abstractions, but I suspect you'd be better off using MockFileSystem here, rather than mocking IFileSystem directly. But basically, IFile.OpenRead isn't declared to return Stream, it's declared to return FileSystemStream, so that's why your mock isn't working... – Jon Skeet Commented Mar 7 at 14:04

Add a comment  | 

1 Answer 1

Sorted by: Reset to default 0

var fileSystem = new System.IO.Abstractions.FileSystem();
// Mock System.IO.Abstractions's FileSystem
var mockFS = new Mock<FileSystem>();
var stream = fileSystem.File.Create("my file path.txt");
mockFS.Setup(x => x.File.OpenRead("my file path.txt")).Returns(stream);

Found this answer posted by Super Jade (A million thanks)

How do I create a FileSystemStream?

However, when I test the production code the instance is null. My file does exist at the filePath..

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