What is the correct way to call DateTime.TryParse

问题:

What is the correct way to call DateTime.TryParse from F#? I am trying to test some code from F# interactive and I can't figure out how to pass a mutable DateTime into the second argument by ref. What is the in/out/ref syntax in F#?

This is the method signature I'm looking at: http://msdn.microsoft.com/en-us/library/ch92fbc1.aspx?cs-save-lang=1&cs-lang=fsharp#code-snippet-1

回答1:

Chris's answer is correct if you really need to pass a mutable DateTime by reference. However, it is much more idiomatic in F# to use the compiler's ability to treat trailing out parameters as tupled return values:

let couldParse, parsedDate = System.DateTime.TryParse("11/27/2012")

Here, the first value is the bool return value, while the second is the assigned out parameter.

回答2:

Here's how to execute DateTime.TryParse in F#:

let mutable dt2 = System.DateTime.Now
let b2 = System.DateTime.TryParse("12-20-04 12:21:00", &dt2)

Where the & operator finds the memory address of dt2 in order to modify the reference.

Here's some additional information on F# parameter syntaxt.

回答3:

Just for the sake of completeness, yet another option is to use ref cells, e.g.

let d = ref System.DateTime.MinValue
if (System.DateTime.TryParse("1/1/1", d)) then
   // ...