Errors and warnings when using lambda expressions and anonymous functions

Article
05/09/2023

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.
CS0834 - A lambda expression must have an expression body to be converted to an expression tree.
CS0835 - Cannot convert lambda to an expression tree whose type argument 'type' is not a delegate type.
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.
CS1951 - An expression tree lambda may not contain an in, out, or ref parameter.
CS1952) - An expression tree lambda may not contain a method with variable arguments.
CS1989 - Async lambda expressions cannot be converted to expression trees.
CS2037 - An expression tree lambda may not contain a COM call with ref omitted on arguments.
CS8030 - Anonymous function converted to a void returning delegate cannot return a value.
CS8072 - An expression tree lambda may not contain a null propagating operator.
CS8074 - An expression tree lambda may not contain a dictionary initializer.
CS8075 - An extension Add method is not supported for a collection initializer in an expression lambda.
CS8153 - An expression tree lambda may not contain a call to a method, property, or indexer that returns by reference.
CS8175 - Cannot use ref local inside an anonymous method, lambda expression, or query expression.
CS8155 - Lambda expressions that return by reference cannot be converted to expression trees.
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.
CS8972 - A lambda expression with attributes cannot be converted to an expression tree.
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) => { };

Conversion to expression trees

Many legal lambda expressions can't be converted into the data structures supported by an expression tree. Attempting to use any of the constructs not supported by expression trees causes one of the following errors:

CS0834 - A lambda expression must have an expression body to be converted to an expression tree.
CS0835 - Cannot convert lambda to an expression tree whose type argument 'type' is not a delegate type.
CS1951 - An expression tree lambda may not contain an in, out, or ref parameter.
CS1952 - An expression tree lambda may not contain a method with variable arguments.
CS1989 - Async lambda expressions cannot be converted to expression trees.
CS2037 - An expression tree lambda may not contain a COM call with ref omitted on arguments.
CS8072 - An expression tree lambda may not contain a null propagating operator.
CS8074 - An expression tree lambda may not contain a dictionary initializer.
CS8075 - An extension Add method is not supported for a collection initializer in an expression lambda.
CS8153 - An expression tree lambda may not contain a call to a method, property, or indexer that returns by reference.
CS8155 - Lambda expressions that return by reference cannot be converted to expression trees.
CS8972 - A lambda expression with attributes cannot be converted to an expression tree.

The general limitations on expressions tree are:

Attributes can't be applied to the lambda expression, its parameters or return.
Statement lambdas aren't allowed. It must be an expression lambda.
The null propagating operator isn't allowed.
Dictionary initializers aren't allowed. Neither are extension Add methods.
async lambda expressions aren't allowed.
in, out, and ref parameters aren't allowed.
ref returns aren't allowed.
Calls to methods that return by ref aren't allowed.
The target expression must be a lambda expression. Constants and variables aren't allowed, but a lambda expression that returns a constant or variable is.
COM calls must include ref on arguments; it can't be implied.
The unsupported __arglist keyword is not allowed.

If you're using any of these constructs in a lambda expression, it can't be converted to an expression tree.