I needed to create a hook into a class I’m working on that will return a GameObject for a given index.  Now, I could have just built that function into my class, but I’d rather make this class re-usable by other projects down the line (and even other classes within the same project).

public System.Func<int,Transform,Transform> rowForRowIndex;

The first two parameters are passed to the function from the calling method, while the third is the returned object.  So in this case, I would invoke the function as follows:

row = rowForRowIndex(r,cachedRow);

Where, r is the index of the row I want to generate, and cachedRow is a row object that has been previously created but is free for use.

You can obviously use different parameters & return type than I am, just remember, the last entry is always the return type, so

public System.Func<bool> isValid;

will return a bool and takes no parameters, while

public System.Func<int,float> rootOfInt;

will return a float and take 1 int as a parameter.

You can also use lambdas to create in-line functions using this syntax:

public System.Func<int,float> rootOfInt = (x) => Mathf.Sqrt(x);

While this is similar to System.Action, the big difference is that System.Func can return a value, System.Action is always type Void.