How to mock an IFormFile for a unit/integration test in ASP.NET Core?

Assuming you have a Controller like..

public class MyController : Controller {
    public Task<IActionResult> UploadSingle(IFormFile file) {...}
}

…where the IFormFile.OpenReadStream() is accessed with the method under test.

As of ASP.NET Core 3.0, use an instance of the FormFile Class which is now the default implementation of IFormFile.

Here is an example of the same test above using FormFile class

[TestClass]
public class IFormFileUnitTests {
    [TestMethod]
    public async Task Should_Upload_Single_File() {
        //Arrange
       
        //Setup mock file using a memory stream
        var content = "Hello World from a Fake File";
        var fileName = "test.pdf";
        var stream = new MemoryStream();
        var writer = new StreamWriter(stream);
        writer.Write(content);
        writer.Flush();
        stream.Position = 0;

        //create FormFile with desired data
        IFormFile file = new FormFile(stream, 0, stream.Length, "id_from_form", fileName);
        
        MyController sut = new MyController();
        
        //Act
        var result = await sut.UploadSingle(file);

        //Assert
        Assert.IsInstanceOfType(result, typeof(IActionResult));
    }
}

Before the introduction of the FormFile Class or in in cases where an instance is not needed you can create a test using Moq mocking framework to simulate the stream data.

[TestClass]
public class IFormFileUnitTests {
    [TestMethod]
    public async Task Should_Upload_Single_File() {
        //Arrange
        var fileMock = new Mock<IFormFile>();
        //Setup mock file using a memory stream
        var content = "Hello World from a Fake File";
        var fileName = "test.pdf";
        var ms = new MemoryStream();
        var writer = new StreamWriter(ms);
        writer.Write(content);
        writer.Flush();
        ms.Position = 0;
        fileMock.Setup(_ => _.OpenReadStream()).Returns(ms);
        fileMock.Setup(_ => _.FileName).Returns(fileName);
        fileMock.Setup(_ => _.Length).Returns(ms.Length);

        var sut = new MyController();
        var file = fileMock.Object;

        //Act
        var result = await sut.UploadSingle(file);

        //Assert
        Assert.IsInstanceOfType(result, typeof(IActionResult));
    }
}