Why Can't I See My C# Windows Form Designer When 'txtbox' is Undeclared?

C# Windows Form Designer cannot be displayed problem

When running the code, the form displays normally, but the designer cannot display it and prompts an error that "txtbox" is not declared or assigned a value.

Understanding Windows Forms Designer

The Windows Forms Designer deserializes and loads the first class in the file to create the component and set its properties based on the InitializeComponents method and the component declaration found in the .cs file.

Cause of designer error

In this example, the declaration of "txtbox" is in the first file, but the designer cannot serialize and load it because it is in a partial class in the first file.

Solution

To fix this problem, just move the declaration of "txtbox" from the first file to a local class in the second file. This allows the designer to access and load the form when it is initialized.

Additional Notes on Windows Forms Designer Behavior

• The base class constructor is executed at design time, while the form class constructor and InitializeComponents are not.
• The designer cannot display forms with abstract base or generic classes.
• Properties defined in your form do not appear in the Properties window, which displays the properties of the base class and the values ​​from your form.
• Form should be the first class in the file for the designer to display it.

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