对VB.NET使用ICOMPARER

Question

I have a List (of Myclass) I want to sort on a property in my class (p propriety AS String) and according to an order determined values Dim myListeValues() AS String = {"ccc", "yyy", "aaa"} I want to use the IComparer interface as:

Public Class MyClass 
   Public Property p As String 

i want do :

Private Class MyComparer Implements Icomparer(of MyClass ) 
Public Function Compare(x AS MyClass ) AS Integer Implements IComparer (Of MyClass ).Compare 

          Return   ???? 


   i want this order : 1) x.p = "ccc" 
                     2) x.p = "yyy" 
                     3) x.p = "aaa"


End FunctionEnd Class


 how can I do that?

Answer 1

Do you know the domain of values that you want to compare? That is, do you know that they will be "ccc", "yyy", and "aaa" (or maybe other values, but all possible values are known ahead of time)?

If the answer is Yes, then I would think about using a Dictionary where the keys are your known values and the values represent the order. Then, in the Compare implementation, retrieve the value for each input string and return the result of comparing them.

Please excuse my poor VB, as I don't use VB.Net and I didn't want to just post a C# version. I think you can get the idea. Essentially you are creating a lookup table that maps your known strings to each string's "ordinal" value (or its position within the set of all known strings). During a Compare of MyClass, get the value of Prop1 (since that looks like what you want to compare) from each of the two MyClass instances that are being compared. Convert each String value to Integer by looking up the string in the dictionary. You can then subtract one Integer from the other to get the compare result.

Some things to consider:

1. Do you want your comparison to be case insensitive (ie "aaa" == "AAA"). If so, you will probably want to use a case insensitive dictionary.

2. You can compare (case insensitive if necessary) the string properties of the two objects to be compared and, if equal, simply return 0. No need to convert to the String values to Integer first.

3. You might want to consider what happens if one (or both) of the MyClass instances being compared has a value that is not in the dictionary (if this can happen).

4. You might want some special handling if the String property of one or both objects is null or empty.

Good luck!

Private Class MyComparer Implements IComparer(of MyClass ) 
    Private Dictionary dict = new Dictionary of (String, Integer) (System.StringComparer.OrdinalIgnoreCase)

    Private Sub New()
        dict.Add("ccc", 0)
        dict.Add("yyy", 1)
        dict.Add("aaa", 2)
    End Sub

    Public Function Compare(x AS MyClass, y AS MyClass ) AS Integer Implements IComparer (Of MyClass ).Compare

        Boolean xb = String.IsNullOrWhitespace(x.Prop1)
        Boolean yb = String.IsNullOrWhitespace(y.Prop1)

        If xb And !yb Then Return -1 'x.Prop1 null or empty, y.Prop1 has a value
        If !xb And yb Then Return 1  'x.Prop1 has a value, y.Prop1 has a value

        If String.Compare(x.Prop1, y.Prop1, StringComparison.OrdinalIgnoreCase) = 0 Then Return 0

        Integer xi = dict[x.Prop1]
        Integer yi = dict[y.Prop1]

        Return xi - yi
    End Function
End Class