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Add errors and warnings for params arrays (#41027)
* Move all params articles into one article Consolidate all related (existing) content into one article. Add new errors and warnings to the introduction. * fix warning * Edit pass to combine different concepts Combine errors on different concepts related to the `params` modifier. * Add final new errors.
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docs/csharp/language-reference/compiler-messages/cs1751.md
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docs/csharp/language-reference/compiler-messages/params-arrays.md
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| --- | ||
| title: Errors and warnings related to `params` arrays | ||
| description: This article helps you diagnose and correct compiler errors and warnings when you use the `params` modifier on method parameters. | ||
| f1_keywords: | ||
| - "CS0225" | ||
| - "CS0231" | ||
| - "CS0466" | ||
| - "CS0674" | ||
| - "CS0758" | ||
| - "CS1104" | ||
| - "CS1611" | ||
| - "CS1670" | ||
| - "CS1751" | ||
| - "CS9218" | ||
| - "CS9219" | ||
| - "CS9220" | ||
| - "CS9221" | ||
| - "CS9222" | ||
| - "CS9223" | ||
| - "CS9224" | ||
| - "CS9225" | ||
| - "CS9227" | ||
| - "CS9228" | ||
| helpviewer_keywords: | ||
| - "CS0225" | ||
| - "CS0231" | ||
| - "CS0466" | ||
| - "CS0674" | ||
| - "CS0758" | ||
| - "CS1104" | ||
| - "CS1611" | ||
| - "CS1670" | ||
| - "CS1751" | ||
| - "CS9218" | ||
| - "CS9219" | ||
| - "CS9220" | ||
| - "CS9221" | ||
| - "CS9222" | ||
| - "CS9223" | ||
| - "CS9224" | ||
| - "CS9225" | ||
| - "CS9227" | ||
| - "CS9228" | ||
| ms.date: 05/20/2024 | ||
| --- | ||
| # 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: | ||
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| <!-- The text in this list generates issues for Acrolinx, because they don't use contractions. | ||
| That's by design. The text closely matches the text of the compiler error / warning for SEO purposes. | ||
| --> | ||
| - [**CS0225**](#parameter-and-argument-type-rules): *The params parameter must be a single-dimensional array or have a valid collection type* | ||
| - [**CS0231**](#method-declaration-rules): *A params parameter must be the last parameter in a formal parameter list.* | ||
| - [**CS0466**](#other-params-errors): *'method1' should not have a params parameter since 'method2' does not* | ||
| - [**CS0674**](#other-params-errors): *Do not use `System.ParamArrayAttribute` or `System.ParamArrayAttribute`/`System.Runtime.CompilerServices.ParamCollectionAttribute`. Use the `params` keyword instead.* | ||
| - [**CS0758**](#other-params-errors): *Both partial method declarations must use a `params` parameter or neither may use a `params` parameter* | ||
| - [**CS1104**](#method-declaration-rules): *A parameter array cannot be used with `this` modifier on an extension method.* | ||
| - [**CS1611**](#method-declaration-rules): *The params parameter cannot be declared as in `ref` or `out`* | ||
| - [**CS1670**](#method-declaration-rules): *`params` is not valid in this context* | ||
| - [**CS1751**](#method-declaration-rules): *Cannot specify a default value for a parameter array.* | ||
| - [**CS9218**](#parameter-and-argument-type-rules): *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**](#parameter-and-argument-type-rules): *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**](#other-params-errors): *Creation of params collection results in an infinite chain of invocation of constructor.* | ||
| - [**CS9224**](#other-params-errors): *Method cannot be less visible than the member with params collection.* | ||
| - [**CS9225**](#other-params-errors): *Constructor leaves required member uninitialized.* | ||
| - [**CS9227**](#parameter-and-argument-type-rules): *Type does not contain a definition for a suitable instance `Add` method.* | ||
| - [**CS9228**](#parameter-and-argument-type-rules): *Non-array params collection type must have an applicable constructor that can be called with no arguments.* | ||
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| In addition, the compiler might produce the following *warning* related to the `params` modifier on method parameters: | ||
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| - [**CS9220**](#parameter-and-argument-type-rules): *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**](#parameter-and-argument-type-rules): *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**](#parameter-and-argument-type-rules): *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.* | ||
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| ## Method declaration rules | ||
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| The following errors indicate using a `params` modifier on a parameter when the `params` modifier isn't allowed in that context: | ||
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| - **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.* | ||
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| The compiler enforces the following rules on your use of the `params` modifier on a method parameter: | ||
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| - 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. | ||
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| In versions before C# 12, the `params` modifier can't be used on the parameter of an anonymous method or lambda expression. | ||
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| ## Parameter and argument type rules | ||
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| The following errors indicate that the type of the parameter used with `params` is invalid: | ||
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| - **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.* | ||
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| 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. | ||
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| - **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.* | ||
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| In versions before C# 13, the `params` modifier is allowed on single-dimensional arrays only. No other types were valid. | ||
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| 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](../operators/collection-expressions.md#conversions). | ||
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| ## Other params errors | ||
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| The following errors indicate other issues with using the `params` modifier: | ||
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| - **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.* | ||
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| 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. | ||
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| You must use the `params` modifier. You can't apply the equivalent attributes, either <xref:System.ParamArrayAttribute?displayProperty=nameWithType> or <xref:System.Runtime.CompilerServices.ParamCollectionAttribute?displayProperty=nameWithType>. | ||
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| The compiler generates one of the final three errors in the preceding list when the code generated to create the collection type is invalid: | ||
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| - 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 <xref:System.Diagnostics.CodeAnalysis.SetsRequiredMembersAttribute?displayProperty=nameWithType> on the parameterless constructor. | ||
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| ## See also | ||
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| - [Extension Methods](../../programming-guide/classes-and-structs/extension-methods.md) | ||
| - [Partial Classes and Methods](../../programming-guide/classes-and-structs/partial-classes-and-methods.md) | ||
| - [Collection expressions](../operators/collection-expressions.md) | ||
| - [params](../keywords/method-parameters.md#params-modifier) |
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