Resolve errors and warnings for deconstruction, discards, and out variables

This article covers the following compiler errors and warnings:

• CS8129: No suitable 'Deconstruct' instance or extension method was found for type 'type', with count out parameters and a void return type.
• CS8130: Cannot infer the type of implicitly-typed deconstruction variable 'variable'.
• CS8131: Deconstruct assignment requires an expression with a type on the right-hand-side.
• CS8132: Cannot deconstruct a tuple of 'element count' elements into 'variable count' variables.
• CS8134: Deconstruction must contain at least two variables.
• CS8136: Deconstruction 'var (...)' form disallows a specific type for 'var'.
• CS8183: Cannot infer the type of implicitly-typed discard.
• CS8187: Tuple element names are not permitted on the left of a deconstruction.
• CS8197: Cannot infer the type of implicitly-typed out variable 'variable'.
• CS8199: The syntax 'var (...)' as an lvalue is reserved.

Deconstruct method requirements

• CS8129: No suitable 'Deconstruct' instance or extension method was found for type 'type', with count out parameters and a void return type.

Provide an accessible instance or extension Deconstruct method that returns void and has one out parameter for each variable on the left. Match each parameter type to the corresponding deconstruction variable (CS8129). For more information, see user-defined deconstruction and the out parameter modifier.

Type inference for deconstruction variables, discards, and out variables

• CS8130: Cannot infer the type of implicitly-typed deconstruction variable 'variable'.
• CS8131: Deconstruct assignment requires an expression with a type on the right-hand-side.
• CS8183: Cannot infer the type of implicitly-typed discard.
• CS8197: Cannot infer the type of implicitly-typed out variable 'variable'.

Supply a typed, deconstructable expression on the right so the compiler can determine each implicitly typed variable (CS8130, CS8131). Cast or otherwise give a discarded expression a type; in a deconstruction, specify an element type when appropriate (CS8183). For an out variable, use a method parameter that supplies the type or specify the type explicitly in the out argument (CS8197). For more information, see deconstruction and calls with out parameters.

Deconstruction cardinality

• CS8132: Cannot deconstruct a tuple of 'element count' elements into 'variable count' variables.
• CS8134: Deconstruction must contain at least two variables.

Use at least two variables in a deconstruction (CS8134). Match the number of variables on the left to the number of tuple elements on the right, and add a discard (_) for each value that you don't need (CS8132). For more information, see discards in tuple and object deconstruction.

Deconstruction declaration and assignment syntax

• CS8136: Deconstruction 'var (...)' form disallows a specific type for 'var'.
• CS8187: Tuple element names are not permitted on the left of a deconstruction.
• CS8199: The syntax 'var (...)' as an lvalue is reserved.

Use var outside the parentheses only for an implicitly typed declaration. Put explicit types on the individual variables instead (CS8136). Remove tuple element names from the left side (CS8187). When assigning existing variables, omit var and use only the parenthesized variables (CS8199):

var (x, y) = point;
(int a, int b) = point;
(a, b) = point;

For more information, see tuple deconstruction.