How to Unit test Textrenderer.DrawText method in C#

Question

public static void DrawText(IDeviceContext dc, string text, Font font, Point pt, Color foreColor, TextFormatFlags flags);

I have a tester application which for my ExtendedComboBox. All the String items given below in code are there in my ComboBox items in my tester Application. How to unit test above method because it returns void? What is the other way to test TextRenderer.Drawtext? Is there any replacement to test OnDrawItem method for drawing the ComboBox Text.

[TestMethod()]
public void ExtendedComboBoxOnDrawPrefixTest()
{
    ExtendedComboBox cboTest = new ExtendedComboBox ();

    // List of strings having special characters.

    string[] items = { 
        "&One",
        "T&wo",
        "E!xclamation",
        "Am@persat",
        "H#ash",
        "Dollar$Sign",
        "Perc%ent",
        "Ci^rcumflex",
        "Ast*erisk",
        "Hy-phen",
        "Und_erscore",
        "pl+us",
        "Equ=als",
        "Col:on",
        "Semi;colon",
        "Co'mma",
        "Inverted\"Comma",
        "Apos'trophe",
        "Pip|e",
        "Open{Brace",
        "Close}Brace",
        "OpenBr[acket",
        "CloseBr]acket",
        "BackS\\lash",
        "ForwardSl/ash",
        "LessT<han",
        "Greate>rThan",
        "Questio?nMark",
        "Do.t",
        "Three",
        "Four",
        "Five",
        "Six",
        "This is a really extremely long string value for a combobox to display."
    };

    cboTest.Items.AddRange(items);

    // To test that all the items have the same kind of prefixes
    for (int index = 0; index < cboTest.Items.Count; index++)
    {
        String expectedText = GetExtendedComboBoxText(cboTest, items[index]);
        Assert.AreEqual(items[index], , String.Format("Item '{0}' returned an string", cboTest.Items[index]));
    }
}

/// <summary>
/// Compare the ComboBoxText of the passed string with respect to the DC, Font, ForeColor and TextFormatFlags.
/// Draw the item 
/// </summary>
private string GetExtendedComboBoxText(Control cboTest, string itemToTest)
{
    TextFormatFlags textFormatflags = TextFormatFlags.NoPrefix;
    Color foreColor = SystemColors.HighlightText;
    return (TextRenderer.DrawText(cboTest.CreateGraphics(), itemToTest, cboTest.Font, new Point(cboTest.Bounds.X, cboTest.Bounds.Y), foreColor, textFormatflags)).Text;
}

Answer 1

It's a BCL method, you shouldn't verify the behavior of a BCL methods.

In UTs there is one assumption; The BCL's methods and classes works correctly. If you verify BCL's methods then you'll have to verify the behavior of int, byte, ToString() and etc...(because you can't trust your infrastructure)

The bottom line you shouldn't verify the behavior of BCL classes(Microsoft already done it for you...). Actually you should assume that the method works correctly(instead of verifying it), then verify that you are using the method with the right parameters.

In order to help my developers, I created a flowchart which demonstrate how to act when they face a such a problem: (In our R&D we found it very usable. we loved the way it affected our R&D...)

In your case, it seems that the expected behavior is viewable, therefore you can verify it through the UI as a part of your Acceptance/Integration/Component tests...

If you still want to verify it as an UT and your test framework doesn't allow you to verify(Rhino Mocks, Moq etc..) this method, you should wrap the method or test the method using an additional tools such MsFakes, Typemock Isolator and etc...

The following snippet demonstrate the way to set an expectation on the method using MsFakes:

    [TestMethod]
    public void PutExpectationOnDrawText()
    {
        var wasExcute = false;
        System.Windows.Forms.Fakes.ShimTextRenderer
                      .DrawTextIDeviceContextStringFontRectangleColorTextFormatFlags =
            (context, txt, font, pt, forecolor, flags) =>
            {
                //the asserts on the orguments...
                //for example:
                Assert.IsNotNull(context);
                Assert.AreNotEqual(String.Empty, txt);
                //...
                wasExcute = true;
            };


        //execute the method which is use DrawText


        Assert.IsTrue(wasExcute);//verify that the method was execute....
    }