How to Resolve Error "CS1016: Named attribute argument expected" in C#

The Compiler Error CS1016 is a syntax error regarding the usage of Attributes. The message reads: "Named attribute argument expected".

In C#, attributes can accept arguments in two ways:

1. Positional Arguments: These map directly to the attribute class's constructor parameters.
2. Named Arguments: These map to the public properties or fields of the attribute class.

The strict rule for attribute syntax is: All positional arguments must appear before any named arguments. Once you start assigning values to properties (e.g., Name = "Value"), you cannot go back to supplying constructor arguments. If the compiler encounters a value without a name after a named argument, it raises CS1016.

Understanding Attribute Argument Order

When you use an attribute like [MyAttribute(1, Prop = 2)]:

• 1 is passed to the constructor.
• Prop = 2 sets the property Prop on the instance after creation.

The compiler parses the argument list from left to right.

• Phase 1: Collect arguments for the constructor.
• Phase 2: Once a named argument (Prop = ...) is seen, switch to "Property Setting Mode".

If you try to pass a constructor argument during "Phase 2", the compiler doesn't know what to do with it because it is expecting another Name = Value pair.

Scenario: Mixing Positional and Named Arguments

This error typically happens when a developer remembers a parameter they forgot to add to the constructor, but adds it to the end of the list after optional property assignments.

Setup

Let's define a custom attribute with one constructor parameter (id) and one optional property (Role).

using System;

[AttributeUsage(AttributeTargets.Class)]
public class DeveloperAttribute : Attribute
{
    // Named Argument (Property)
    public string Role { get; set; }

    // Positional Argument (Constructor)
    public DeveloperAttribute(int id)
    {
        // ...
    }
}

Example of error

Attempting to put the id (101) after setting the Role.

// ⛔️ Error CS1016: Named attribute argument expected
// The compiler sees 'Role = "Backend"'. It expects all subsequent arguments
// to also be named (e.g., Level = 5).
// Instead, it found '101', which is a positional value.
[Developer(Role = "Backend", 101)] 
public class MyService
{
}

Solution: Reorder the Arguments

To fix this, simply move all constructor (positional) arguments to the beginning of the list.

Solution: move 101 to the front.

// ✅ Correct: Positional arguments come first.
// 1. Constructor(101) is called.
// 2. Property Role is set to "Backend".
[Developer(101, Role = "Backend")]
public class MyService
{
}

Conclusion

CS1016 is a simple ordering rule.

1. Identify the Syntax: Look at your attribute usage [Attribute(...)].
2. Check Order: Ensure that values appearing alone (e.g., "Value", 123) appear before values with assignments (e.g., Name = "Value").
3. Refactor: Move standard arguments to the left, property assignments to the right.