C# 2026 - properties all giving me error : does not configure the code serialization for its property content

adding

[DesignerSerializationVisibility(DesignerSerializationVisibility.Visible)] 
[DefaultValue(false)]

in front of my properties resolved the issue.

Hello @Christ Kennedy ,

Thank you for contact Microsoft Q&A. After review your description, I will clarify why the compilation error occurred and summarize the solution below:

• Importing old .cs files into a C# 2026 project caused compilation errors for all properties:

Property 'AllowDecimal' does not configure the code serialization for its property content

• Example failing code:

bool bolAllowDecimal = false;

public bool AllowDecimal

{

    get { return bolAllowDecimal; }

    set { bolAllowDecimal = value; }

}

This happened because the designer in C# 2026 (Windows App SDK) looks for serialization attributes on the property itself. In the imported code the attributes were applied to the backing field, so the designer ignored them and couldn't configure property serialization which caused the compilation error.

• In C# 2026 with Windows App SDK, the designer requires explicit serialization attributes on properties, not on backing fields.
• Initial attempts applied [DesignerSerializationVisibility] and [DefaultValue] to the field instead of the property, so the compiler could not configure serialization.

As I can see your solution had worked perfectly:

• Apply attributes directly to the property:

[DesignerSerializationVisibility(DesignerSerializationVisibility.Visible)]

[DefaultValue(false)]

public bool AllowDecimal

{

    get { return bolAllowDecimal; }

    set { bolAllowDecimal = value; }

}

• After this change, your project compiled successfully.

You can read more information about property serialization in Windows App SDK here:

• DesignerSerializationVisibility Attribute
• DefaultValue Attribute

I hope this explanation clarifies the root cause of the compilation errors. If you have any further questions, feel free to ask!