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Resolve errors and warnings related to local functions

This article covers the following compiler errors and warnings:

  • CS8108: Cannot pass argument with dynamic type to params parameter 'parameter' of local function 'method'.
  • CS8112: Local function 'method' must declare a body because it is not marked 'static extern'.
  • CS8321: The local function 'method' is declared but never used
  • CS8421: A static local function cannot contain a reference to 'variable'.
  • CS8422: A static local function cannot contain a reference to 'this' or 'base'.

Local functions were added in C# 7.0. They let you declare helper methods inside another member. The diagnostics in this article cover local function declarations, calls, usage, and the extra capture restrictions for static local functions.

Local function bodies

  • CS8112: Local function 'method' must declare a body because it is not marked 'static extern'.

A local function declaration must include a body unless the local function is declared static extern. Add a block body or an expression body to the local function. If the implementation is supplied externally, mark the local function static extern and ensure the declaration follows the rules for external methods.

Params parameters and dynamic arguments

  • CS8108: Cannot pass argument with dynamic type to params parameter 'parameter' of local function 'method'.

A local function call can't pass a dynamic argument to a params parameter. Convert the value to the element type or array type expected by the params parameter before the call. You can also store the values in an explicitly typed array and pass that array to the local function.

Unused local functions

  • CS8321: The local function 'method' is declared but never used

The compiler reports this warning when you declare a local function but no reachable code calls it. Remove the unused local function, or call it from the enclosing member. If you intend to use the local function in a future change, keep it out of the source until it's needed.

Static local functions can't capture state

  • CS8421: A static local function cannot contain a reference to 'variable'.
  • CS8422: A static local function cannot contain a reference to 'this' or 'base'.

A static local function can't capture state from the enclosing scope. It can't reference enclosing local variables, parameters, instance members through this, or this itself. Pass each value the local function needs as a parameter (CS8421, CS8422). If the local function must capture variables or instance state from the enclosing scope, remove the static modifier.

The following example shows references that cause CS8422:

public class C
{
    private int counter = 1;

    public void IncreaseCounter()
    {
        static void LocalFunc(int addition)
        {
            this.counter += addition;   // CS8422
            base.ToString();            // CS8422

            counter += addition;        // CS8422: implicit this
            ToString();                 // CS8422: implicit base
        }

        LocalFunc(10);
    }
}

To fix CS8422, pass the required instance data to the local function, move the helper outside the instance context, or remove static when the local function must access instance state.

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