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Errors and warnings related to the params modifier on method parameters

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There are a few errors related to the lock statement and thread synchronization:

  • CS0225: The params parameter must be a single-dimensional array or have a valid collection type
  • CS0231: A params parameter must be the last parameter in a formal parameter list.
  • CS0466: 'method1' should not have a params parameter since 'method2' does not
  • CS0674: Do not use System.ParamArrayAttribute or System.ParamArrayAttribute/System.Runtime.CompilerServices.ParamCollectionAttribute. Use the params keyword instead.
  • CS0758: Both partial method declarations must use a params parameter or neither may use a params parameter
  • CS1104: A parameter array cannot be used with this modifier on an extension method.
  • CS1611: The params parameter cannot be declared as in ref or out
  • CS1670: params is not valid in this context
  • CS1751: Cannot specify a default value for a parameter array.
  • CS9218: The type arguments for method cannot be inferred from the usage because an argument with dynamic type is used and the method has a non-array params collection parameter. Try specifying the type arguments explicitly.
  • CS9219: Ambiguity between expanded and normal forms of non-array params collection parameter of, the only corresponding argument has the type dynamic. Consider casting the dynamic argument.
  • CS9223: Creation of params collection results in an infinite chain of invocation of constructor.
  • CS9224: Method cannot be less visible than the member with params collection.
  • CS9225: Constructor leaves required member uninitialized.
  • CS9227: Type does not contain a definition for a suitable instance Add method.
  • CS9228: Non-array params collection type must have an applicable constructor that can be called with no arguments.

In addition, the compiler might produce the following warning related to the params modifier on method parameters:

  • CS9220: One or more overloads of method having non-array params collection parameter might be applicable only in expanded form which is not supported during dynamic dispatch.
  • CS9221: One or more indexer overloads having non-array params collection parameter might be applicable only in expanded form which is not supported during dynamic dispatch.
  • CS9222: One or more constructor overloads having non-array params collection parameter might be applicable only in expanded form which is not supported during dynamic dispatch.

Method declaration rules

The following errors indicate using a params modifier on a parameter when the params modifier isn't allowed in that context:

  • CS0231: A params parameter must be the last parameter in a formal parameter list.
  • CS1104: A parameter array cannot be used with this modifier on an extension method.
  • CS1611: The params parameter cannot be declared as in ref or out
  • CS1670: params is not valid in this context
  • CS1751: Cannot specify a default value for a parameter array.

The compiler enforces the following rules on your use of the params modifier on a method parameter:

  • The params modifier is allowed only on the last parameter in a formal parameter list. This includes any parameters with a default value.
  • You can't include a default argument for the parameter when the params modifier is used.
  • The params modifier can't be applied to reference parameter. A reference parameter is one with the in, ref readonly, ref or out modifier.
  • The params modifier can't be combined with the this modifier on an extension method.
  • The params modifier can't be used on an overloaded operator.

In versions before C# 12, the params modifier can't be used on the parameter of an anonymous method or lambda expression.

Parameter and argument type rules

The following errors indicate that the type of the parameter used with params is invalid:

  • CS9218: The type arguments for method cannot be inferred from the usage because an argument with dynamic type is used and the method has a non-array params collection parameter. Try specifying the type arguments explicitly.
  • CS9219: Ambiguity between expanded and normal forms of non-array params collection parameter of, the only corresponding argument has the type 'dynamic'. Consider casting the dynamic argument.
  • CS0225: The params parameter must be a single-dimensional array or have a valid collection type
  • CS9227: Type does not contain a definition for a suitable instance Add method.
  • CS9228: Non-array params collection type must have an applicable constructor that can be called with no arguments.

The following warnings indicate that the one of the possible overloads might involve dynamic dispatch. The dynamic nature of the argument doesn't participate in overload resolution.

  • CS9220: One or more overloads of method having non-array params collection parameter might be applicable only in expanded form which is not supported during dynamic dispatch.
  • CS9221: One or more indexer overloads having non-array params collection parameter might be applicable only in expanded form which is not supported during dynamic dispatch.
  • CS9222: One or more constructor overloads having non-array params collection parameter might be applicable only in expanded form which is not supported during dynamic dispatch.

In versions before C# 13, the params modifier is allowed on single-dimensional arrays only. No other types were valid.

Starting with C# 13 any valid collection type can be used. However, some restrictions remain. The collection type must follow the same rules as the target of a collection expression.

Other params errors

The following errors indicate other issues with using the params modifier:

  • CS0466: 'method1' should not have a params parameter since 'method2' does not
  • CS0674: Do not use System.ParamArrayAttribute or System.Runtime.CompilerServices.ParamCollectionAttribute. Use the params keyword instead.
  • CS0758: Both partial method declarations must use a params parameter or neither may use a params parameter
  • CS9223: Creation of params collection results in an infinite chain of invocation of constructor.
  • CS9224: Method cannot be less visible than the member with params collection.
  • CS9225: Constructor leaves required member uninitialized.

A method that implements an interface must include the params modifier if and only if the interface member has the params modifier. Similarly, either both declarations of a partial method must include the params modifier, or none can include the params modifier.

You must use the params modifier. You can't apply the equivalent attributes, either System.ParamArrayAttribute or System.Runtime.CompilerServices.ParamCollectionAttribute.

The compiler generates one of the final three errors in the preceding list when the code generated to create the collection type is invalid:

  • The compiler emits CS9223 when the code emitted to create the collection also contains a params collection of the same type. Typically, the Add method takes a params collection of the same type.
  • The compiler emits CS9224 when the Create method for the collection type is less accessible than the method that takes the params parameter of the collection type.
  • The compiler emits CS9225 when the collection type has a required member and the parameterless constructor doesn't initialize that member and have the System.Diagnostics.CodeAnalysis.SetsRequiredMembersAttribute on the parameterless constructor.

See also