Moq example for SO.cs

// https://stackoverflow.com/questions/55498470/mocking-streamreader-in-c-sharp-for-unit-test/
public static void Main()
{
    Mock<IFileManager> mockFileManager = new Mock<IFileManager>();
    string fakeFileContents = "Hello world";
    byte[] fakeFileBytes = Encoding.UTF8.GetBytes(fakeFileContents);
    
    MemoryStream fakeMemoryStream = new MemoryStream(fakeFileBytes);

    mockFileManager.Setup(fileManager => fileManager.StreamReader(It.IsAny<string>()))
                   .Returns(() => new StreamReader(fakeMemoryStream));
    
    Foo foo = new Foo(mockFileManager.Object);
    
    string result = foo.ParseFile("test.txt");
    
    Console.WriteLine(result);
}

public interface IFileManager
{
    StreamReader StreamReader(string path);
}

public class Foo
{
    IFileManager fileManager;

    public Foo(IFileManager fileManager)
    {
        this.fileManager = fileManager;
    }

    public string ParseFile(string filePath)
    {
        using (var reader = fileManager.StreamReader(filePath))
        {
            return reader.ReadToEnd();
        }
    }
}