S4260 — "ConstructorArgument" parameters should exist in constructors

Why is this an issue?

When creating a custom Markup Extension that accepts parameters in WPF, the ConstructorArgument markup must be used to identify the discrete properties that match these parameters. However since this is done via a string, the compiler won’t give you any warning in case there are typos.

This rule raises an issue when the string argument to ConstructorArgumentAttribute doesn’t match any parameter of any constructor.

How to fix it

Code examples

Noncompliant code example

using System;

namespace MyLibrary
{
  public class MyExtension : MarkupExtension
  {
    public MyExtension() { }

    public MyExtension(object value1)
    {
      Value1 = value1;
    }

    [ConstructorArgument("value2")]   // Noncompliant
    public object Value1 { get; set; }
  }
}

Compliant solution

using System;

namespace MyLibrary
{
  public class MyExtension : MarkupExtension
  {
    public MyExtension() { }

    public MyExtension(object value1)
    {
      Value1 = value1;
    }

    [ConstructorArgument("value1")]
    public object Value1 { get; set; }
  }
}

Resources

Documentation

• Microsoft Learn - Markup Extensions and WPF XAML
• Microsoft Learn - MarkupExtension Class
• Microsoft Learn - ConstructorArgumentAttribute Class