Understanding the Problem: Default Property Values in Inherited Controls

When working with inherited controls in Visual Studio, setting default property values can be a common challenge. This often occurs when you want your custom control to have a new default value, but the base class defines a different default. For instance, let’s say you create a custom combo box control derived from System.Windows.Forms.ComboBox and want the DropDownItems property to default to 50 instead of its original value.

The Issue at Hand

When you derive a class and set the property in your constructor, like this:

class NewCombo : System.Windows.Forms.ComboBox
{
    public NewCombo() { DropDownItems = 50; }
}

Upon dragging the control into a form, Visual Studio automatically generates a designer file code like mycontrol.DropDownItems = 50;. This isn’t inherently problematic until you decide to change that value later. If you update the constructor to:

DropDownItems = 45;

You still face the issue that all previously placed controls will retain their hard-coded value of 50 in the designer files. This goes against the purpose of having a single source for your default values, thus complicating maintenance and future updates.

Finding a Solution: Overriding Properties and Applying Attributes

To tackle this issue efficiently, you can either override or shadow the property in your derived class and re-apply the default value attribute. Here’s how you can achieve this:

Step-by-Step Guide

1. Define a New Property

To override the default DropDownItems property effectively, follow these steps in your custom control class:

class NewCombo : System.Windows.Forms.ComboBox
{
    [System.ComponentModel.DesignerSerializationVisibility(System.ComponentModel.DesignerSerializationVisibility.Visible)]
    public new int DropDownItems
    {
        get { return base.DropDownItems; }
        set { base.DropDownItems = value; }
    }

    public NewCombo()
    {
        DropDownItems = 50; // Set your new default value
    }
}

2. Annotate with DesignerSerializationVisibility

By utilizing the DesignerSerializationVisibility attribute, you affect how properties are serialized in the designer. In this case:

• Visible indicates that this property should be visible in the designer.
• Setting it as new allows you to hide the base implementation effectively.

3. Control Behavior in Designer

This approach ensures that each instance of NewCombo respects the default value you’ve set in the constructor, allowing for easy updates should the need arise.

Key Takeaways

• Override/Shadow Property: Use the new keyword to create your property and set the default value as required.
• Designer Attributes: Utilize attributes such as DesignerSerializationVisibility to dictate how properties behave in Visual Studio’s designer.

Conclusion

By following these techniques, you can efficiently manage default property values for inherited controls in Visual Studio. This not only simplifies your development process but also ensures that your code remains maintainable and adaptable to changes requested by clients or project requirements.

Now, you can enjoy the flexibility of having centralized default properties without worrying about hard-coded values in designer files!