Moq (and other DynamicProxy-based mocking frameworks) are unable to mock anything that is not a virtual or abstract method.
Sealed/static classes/methods can only be faked with Profiler API based tools, like Typemock (commercial) or Microsoft Moles (free, known as Fakes in Visual Studio 2012 Ultimate /2013 /2015).
Alternatively, you could refactor your design to abstract calls to static methods, and provide this abstraction to your class via dependency injection. Then you'd not only have a better design, it will be testable with free tools, like Moq.
A common pattern to allow testability can be applied without using any tools altogether. Consider the following method:
public class MyClass
{
public string[] GetMyData(string fileName)
{
string[] data = FileUtil.ReadDataFromFile(fileName);
return data;
}
}
Instead of trying to mock FileUtil.ReadDataFromFile, you could wrap it in a protected virtual method, like this:
public class MyClass
{
public string[] GetMyData(string fileName)
{
string[] data = GetDataFromFile(fileName);
return data;
}
protected virtual string[] GetDataFromFile(string fileName)
{
return FileUtil.ReadDataFromFile(fileName);
}
}
Then, in your unit test, derive from MyClass and call it TestableMyClass. Then you can override the GetDataFromFile method to return your own test data.
Hope that helps.