Taking the following simple method, how would you suggest I write a unit test for it (I am using MSTest however concepts are similar in other tools).

public void MyMethod(MyObject myObj, bool validInput)
{    
 if(!validInput)
 {
  // Do nothing
 }
 else
 {
  // Update the object
  myObj.CurrentDateTime = DateTime.Now;
  myObj.Name = "Hello World";
 }
}

If I try and follow the rule of one assert per test, my logic would be that I should have a Class Initialise method which executes the method and then individual tests which check each property on myobj.

public class void MyTest
{
    MyObj myObj;

    [TestInitialize]
    public void MyTestInitialize()
    {
        this.myObj = new MyObj();
        MyMethod(myObj, true);
    }

    [TestMethod]
    public void IsValidName()
    {
        Assert.AreEqual("Hello World", this.myObj.Name);
    }

    [TestMethod]
    public void IsDateNotNull()
    {
        Assert.IsNotNull(this.myObj.CurrentDateTime);
    }
}

Where I am confused is around the TestInitialize. If I execute the method under TestInitialize, I would need seperate classes per variation of parameter inputs. Is this correct? This would leave me with a huge number of files in my project (unless I have multiple classes per file).

Thanks