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
{
}

note

Why is this strict? Unlike standard method calls (where C# 4.0+ allows named arguments in any order), Attributes are metadata stored in the assembly. The compiler must rigorously distinguish between what is required for instantiation (constructor) and what is optional configuration (properties) to serialize the metadata correctly.

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.