Errors and warnings related to extension methods declared with this parameters or extension blocks

• CS1100: Method has a parameter modifier 'this' which is not on the first parameter
• CS1101: The parameter modifier 'ref' cannot be used with 'this'.
• CS1102: The parameter modifier 'out' cannot be used with 'this'.
• CS1103: The first parameter of an extension method cannot be of a pointer type.
• CS1105: Extension methods must be static.
• CS1106: Extension methods must be defined in a non generic static class.
• CS1109: Extension Methods must be defined on top level static classes, 'name' is a nested class.
• CS1110: Cannot define a new extension because the compiler required type ExtensionAttribute cannot be found. Are you missing a reference to System.Core.dll?
• CS1112: Do not use 'ExtensionAttribute'. Use the 'this' keyword instead.
• CS1113: Extension method defined on a value type cannot be used to create delegates.
• CS1743: Cannot specify a default value for the 'this' parameter.
• CS9281: Extension declarations may not have a name.
• CS9282: Extension declarations can include only methods or properties.
• CS9283: Extensions must be declared in a top-level, non-generic, static class.
• CS9284: The receiver parameter of an extension cannot have a default value.
• CS9285: An extension container can have only one receiver parameter.
• CS9287: A receiver parameter cannot have the same name as an extension container type parameter.
• CS9288: A parameter, local variable, or local function cannot have the same name as an extension container type parameter.
• CS9289: Member type parameter has the same name as an extension container type parameter.
• CS9290: A parameter, local variable, or local function cannot have the same name as an extension parameter.
• CS9291: 'value': an automatically-generated parameter name conflicts with an extension parameter name.
• CS9292: A type parameter has the same name as an extension parameter.
• CS9293: Cannot use an extension parameter in this context.
• CS9294: 'value': an automatically-generated parameter name conflicts with an extension type parameter name.
• CS9295: The extended type must reference all the type parameters declared by the extension, but a type parameter is not referenced.
• CS9300: The 'ref' receiver parameter of an extension block must be a value type or a generic type constrained to struct.
• CS9301: The 'in' or 'ref readonly' receiver parameter of extension must be a concrete (non-generic) value type.
• CS9302: new protected member declared in an extension block.
• CS9303: Cannot declare instance members in an extension block with an unnamed receiver parameter.
• CS9304: Cannot declare init-only accessors in an extension block.
• CS9305: Cannot use modifiers on the unnamed receiver parameter of extension block.

Common errors on extension declarations

The compiler emits these errors when you violate rules that apply to all extension member declarations, regardless of the syntax chosen:

• CS1102: The parameter modifier 'out' cannot be used with 'this'.
• CS1106: Extension methods must be defined in a non generic static class.
• CS1103: The first parameter of an extension method cannot be of a pointer type.
• CS1109: Extension Methods must be defined on top level static classes, 'name' is a nested class.
• CS1113: Extension method defined on a value type cannot be used to create delegates.
• CS1743: Cannot specify a default value for the 'this' parameter.
• CS9283: Extensions must be declared in a top-level, non-generic, static class.
• CS9284: The receiver parameter of an extension cannot have a default value.
• CS9285: An extension container can have only one receiver parameter.

Any extension declaration must follow these rules:

• Its containing type (class or struct) must be non-generic and static.
• Its containing type must be a top-level type. It can't be nested in another type.
• Members that extend an instance of a value type can't be converted to delegates.
• The receiver parameter can't include the out parameter modifier.
• The receiver parameter can't have a default argument value.
• Pointer types can't be extended. In other words, the parameter you apply the this modifier to can't be a pointer type.

Errors related to extension block declarations

These errors are specific to extension blocks, a C# 14 feature. Extension blocks are declared using the extension keyword in a static class. The extension declares the type and name of the receiver. All members inside the block declared with extension are extension members for that receiver:

• CS9281: Extension declarations may not have a name.
• CS9282: Extension declarations can include only methods or properties.
• CS9287: A receiver parameter cannot have the same name as an extension container type parameter.
• CS9288: A parameter, local variable, or local function cannot have the same name as an extension container type parameter.
• CS9289: Member type parameter has the same name as an extension container type parameter.
• CS9290: A parameter, local variable, or local function cannot have the same name as an extension parameter.
• CS9291: 'value': an automatically-generated parameter name conflicts with an extension parameter name.
• CS9292: A type parameter has the same name as an extension parameter.
• CS9293: Cannot use an extension parameter in this context.
• CS9294: 'value': an automatically-generated parameter name conflicts with an extension type parameter name.
• CS9295: The extended type must reference all the type parameters declared by the extension, but a type parameter is not referenced.
• CS9300: The 'ref' receiver parameter of an extension block must be a value type or a generic type constrained to struct.
• CS9301: The 'in' or 'ref readonly' receiver parameter of extension must be a concrete (non-generic) value type.
• CS9302: new protected member declared in an extension block.
• CS9303: Cannot declare instance members in an extension block with an unnamed receiver parameter.
• CS9304: Cannot declare init-only accessors in an extension block.
• CS9305: Cannot use modifiers on the unnamed receiver parameter of extension block.

Extension declarations must follow these rules:

• The extension can't include a name token. The extension declares the receiver only.
• The receiver parameter can't have a default value.

Extension members declared in an extension block must follow these rules, in addition to the common rules:

• Only methods and properties are valid extension member types. Extension members can extend an instance, or a type.
• The extension must provide a parameter name for the receiver in order to contain members that extend an instance.
• The receiver parameter name must be unique in that extension block.
• All extension members must use all type parameters declared on the extension. They can add more type parameters.

Errors related to this parameter extension methods

These errors are specific to extension methods where you declare the receiver by adding the this modifier to the first parameter of the method:

• CS1100: Method has a parameter modifier 'this' which is not on the first parameter
• CS1101: The parameter modifier 'ref' cannot be used with 'this'.
• CS1105: Extension methods must be static.
• CS1110: Cannot define a new extension because the compiler required type ExtensionAttribute cannot be found. Are you missing a reference to System.Core.dll?
• CS1112: Do not use 'ExtensionAttribute'. Use the 'this' keyword instead.

An extension method where the receiver instance includes the this modifier must follow these rules, in addition to the common rules:

• The method must have the static modifier.
• The this parameter modifier must be applied to the first parameter. It can't be applied to any other parameters on the method.
• The ref out parameter modifier can't be applied to the first parameter. To apply ref, you need to convert to an extension block.
• In .NET Framework apps, System.Core.dll must be added as a reference.
• You must specify the this modifier on the first parameter. You can't directly use the ExtensionAttribute attribute instead.