Doesn’t C# Extension Methods allow passing parameters by reference?

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No. In C#, you cannot specify any modifiers (like ‘out’ or ref) other than this for the first parameter of an extension method – you can for the others. Not familiar with the VB Syntax but it seems to be using a declarative approach to mark an extension method.

When you call it, you do not specify the first this parameter. Hence marking the parameter as out or ref doesnt make sense as You can’t specify the modifier when you call it like you’d do for normal methods

void MyInstanceMethod(ref SomeClass c, int data) { ... } // definition

obj.MyInstanceMethod(ref someClassObj, 10);              // call

void MyExtensionMethod(this SomeClass c, int data) {.... } // defn

c.MyExtensionMethod(10);                                 // call

I think the trouble you’re having here is related to value types being immutable. If Weekdays was a reference type, it would work out alright. For immutable types (structs), the defacto way is to return a new instance with the required value. E.g. See the Add method on the struct DateTime, it returns a new DateTime instance whose value = receiver DateTime instance’s value + param value.

public DateTime Add( TimeSpan value )

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