Errors and warnings when using lambda expressions and anonymous functions

There are several errors related to declaring and using lambda expressions:

• CS0748: Inconsistent lambda parameter usage; parameter types must be all explicit or all implicit.
• CS1621: The yield statement cannot be used inside an anonymous method or lambda expression.
• CS1628: Cannot use in ref or out parameter inside an anonymous method, lambda expression, or query expression.
• CS1632: Control cannot leave the body of an anonymous method or lambda expression.
• CS1673: Anonymous methods, lambda expressions, and query expressions inside structs cannot access instance members of 'this'.
• CS1686: Local variable or its members cannot have their address taken and be used inside an anonymous method or lambda expression.
• CS1706: Expression cannot contain anonymous methods or lambda expressions.
• CS8030: Anonymous function converted to a void returning delegate cannot return a value.
• CS8175: Cannot use ref local inside an anonymous method, lambda expression, or query expression.
• CS8916: Attributes on lambda expressions require a parenthesized parameter list.
• CS8971: InterpolatedStringHandlerArgument has no effect when applied to lambda parameters and will be ignored at the call site.
• CS8975: The contextual keyword var cannot be used as an explicit lambda return type.
• CS9098: Implicitly typed lambda parameter '...' cannot have a default value.

In addition, there are several warnings related to declaring and using lambda expressions:

• CS8971: InterpolatedStringHandlerArgument has no effect when applied to lambda parameters and will be ignored at the call site.
• CS9099: The default parameter value does not match in the target delegate type.
• CS9100: Parameter has params modifier in lambda but not in target delegate type.

Syntax limitations in lambda expressions

Some C# syntax is prohibited in lambda expressions and anonymous methods. Using invalid constructs in a lambda expression causes the following errors:

• CS1621: The yield statement cannot be used inside an anonymous method or lambda expression.
• CS1628: Cannot use in, ref, or out parameter inside an anonymous method, lambda expression, or query expression.
• CS1632: Control cannot leave the body of an anonymous method or lambda expression.
• CS1673: Anonymous methods, lambda expressions, and query expressions inside structs cannot access instance members of this.
• CS1686: Local variable or its members cannot have their address taken and be used inside an anonymous method or lambda expression.
• CS8175: Cannot use ref local inside an anonymous method, lambda expression, or query expression.

All the following constructs are disallowed in lambda expressions:

• yield statements (yield return or yield break)
• Calling a method that has an in, ref, or out parameter
• ref local variables
• break, goto, and continue statements
• this access when this is a struct type
• Anonymous methods or lambda expressions inside another expression, such as an Attribute constructor.

You can't use any of these constructs in a lambda expression or an anonymous method. Many are allowed in a local function.

In addition, interpolated string handler types are ignored when applied to a lambda parameter. If you use one, you see the following warning:

• CS8971: InterpolatedStringHandlerArgument has no effect when applied to lambda parameters and will be ignored at the call site.

Lambda expression parameters and returns

These errors indicate a problem with a parameter declaration:

• CS0748: Inconsistent lambda parameter usage; parameter types must be all explicit or all implicit.
• CS9098: Implicitly typed lambda parameter '...' cannot have a default value.
• CS8030: Anonymous function converted to a void returning delegate cannot return a value.
• CS8916: Attributes on lambda expressions require a parenthesized parameter list.
• CS8975: The contextual keyword 'var' cannot be used as an explicit lambda return type.

Lambda expression parameters must follow these rules:

• When a lambda expression has multiple parameters, either all parameters must be explicitly typed or all parameters must be implicitly typed.
• All lambda parameters with a default value must be explicitly typed.
• If attributes are applied to any parameters, the parameter list must be enclosed in parentheses.

Return types of lambda expression must follow these rules:

• A lambda expression that returns any value can't be converted to a void returning delegate, such as Action.
• The return type is either inferred, or an explicit type. A return type declared using the keyword var isn't allowed.

Lambda expression delegate type

• CS9099: Warning: The default parameter value does not match in the target delegate type.

When you declare a default value or add the params modifier with a lambda expression parameter, the delegate type isn't one of the Func or Action types. Rather, it's a custom type that includes the default parameter value or params modifier. The following code generates warnings because it assigns a lambda expression that has a default parameter to an Action type:

Action<int> a1 = (int i = 2) => { };
Action<string[]> a3 = (params string[] s) => { };

To fix the error, either remove the default parameter or use an implicitly typed variable for the delegate type:

Action<int> a1 = (int i) => { };
var a2 = (int i = 2) => { };
Action<string[]> a3 = (string[] s) => { };
var a4 = (params string[] s) => { };