Enforce non-void returning methods to always assign to a variable or discard

Often I have the pieces of code that look like this:

        private void OnChangeLanguageCommandExecuted(object obj)
        {
            pLngService.ChangeLanguage(newLcid);
        }

In this case the ChangeLanguage(...) method returns a value, although that value (true for success, false for non-success) is not used. The problem is, I had no clue when looking at the code that this method was returning a value.

I did not write the method, nor have I control over it.

Therefore i'd like to globally enforce a policy where:

Each non-void returning method must be assigned a variable, e.g. var unused = pLngService.ChangeLanguage(newLcid);

Or the discard operator should be used, to make it more explicit: _ = pLngService.ChangeLanguage(newLcid);

I'm of course open to other suggestions, the main goal here is to make it more verbose that both a method is returning a value and that i'm choosing to discard it.

I was hoping there'd be a rule for visual studio or Resharper where i could enfore this policy by generating compiler warnings. I won't like to make this a compiler error, that seems to rigorous. I did some quick looking around but i did not find anything oob but i feel like im overlooking something.

I'm using projects in vs2017 (net4) and vs2019 (net8/netcore3.0) so something that would work in either of those setups would be great.

EDIT: I found out, literally whilst writing a roslyn code analyzer, that apperently you can configure this with https://docs.microsoft.com/en-us/dotnet/fundamentals/code-analysis/style-rules/ide0058

The code fixes were exactly what I was looking for:

// Original code:
System.Convert.ToInt32("35");

// After code fix for IDE0058:

// csharp_style_unused_value_expression_statement_preference = discard_variable
_ = System.Convert.ToInt32("35");

// csharp_style_unused_value_expression_statement_preference = unused_local_variable
var unused = Convert.ToInt32("35");

There is the following rule:

csharp_style_unused_value_expression_statement_preference

With options:

Option values discard_variable - Prefer to assign an unused expression to a discard

unused_local_variable - Prefer to assign an unused expression to a local variable that is never used