c# - Passing delegate function with extra parameters

Question

I have a delegate which looks like the following:

public delegate bool ApprovalPrompt(ApprovalType type, int receipt, params string[] info);

I accept a delegate of this type as a parameter to the function I want to call. However, in one particular calling function, I want to pass some extra data to the function which matches this delegate.

Here's the signature of the implementing function:

private static bool LogApprovalNeeded(FraudFilterUtilities.ApprovalType type, int receipt, params string[] info)

and it's being called as follows:

PrepareReceipt(LogApprovalNeeded);

I'd like it to be:

private static bool LogApprovalNeeded(Customer cust, FraudFilterUtilities.ApprovalType type, int receipt, params string[] info)

which ideally would be used as follows:

PrepareReceipt(LogApprovalNeeded(myCustomer))

---

How can I accomplish such a thing? I'd rather not need to declare a field in the class just to hold the Customer parameter between one function and the callback...

Answer 1

You can use a lambda to "curry" your function:

PrepareReceipt((type, receipt, info) => 
    LogApprovalNeeded(myCustomer, type, receipt, info))

Currying a function is the formal term for storing a reference to a function but with one or more of the parameters "fixed", thus altering the signature of the method.

You can also use a lambda when the signature of your function doesn't need all of the arguments that the delegate is providing; you can effectively discard parameters by not passing forward all of the arguments in the lambda.