How to iterate through a property of a class in VB

问题:

If I have a class object A, and it has properties such as a0, a1, a2... If this class has 100 properties like this (up to a99). I would like to display each of these properties, but I do not want to have 100 lines of code of calling this as following

print A.a0
print A.a1
print A.a2
...
print A.a99

The code is too inefficient, so I am wondering if there is a way to loop through these properties. Thank you.

回答1:

.NET provides the ability to examine an object at runtime through a process known as reflection. The purpose of the original post was to iterate through an object's properties in an automated fashion rather than by manually coding explicit statements that displayed each property, and reflection is a process to accomplish this very thing.

For this particular purpose, looping through an object's properties at run-time, you use the GetProperties() method that is available for each Type. In your case, the Type you want to "reflect" is A, so the type-specific version of GetProperties returns a list of the instance properties for that object.

When you ask .NET to return the properties of an object, you can also specify what's called a binding flag that tells .NET which properties to return - public properties, private properties, static properties - a myriad of combinations from about twenty different values in the BindingFlags enumeration. For the purposes of this illustration, BindingFlags.Public will suffice, assuming your A0-A999 properties are declared to be public. To expose even more properties, simply combine multiple BindingFlag values with a logical "or".

So, now armed with that information, all we need to do is create a class, declare its properties, and tell Reflection to enumerate the properties for us. Assuming your Class A exists with property names A0-A999 already defined, here's how you'd enumerate ones starting with "A":

// Assuming Class "A" exists, and we have an instance of "A" held in 
// a variable ActualA...
using System.Reflection

// The GetProperties method returns an array of PropertyInfo objects...
PropertyInfo[] properties = typeof(ActualA).GetProperties(BindingFlags.Public);
// Now, just iterate through them.
foreach(PropertyInfo property in properties)
{
    if (property.Name.StartsWith("A")){
    // use .Name, .GetValue methods/props to get interesting info from each property.
        Console.WriteLine("Property {0}={1}",property.Name,
                                             property.GetValue(ActualA,null));
    }
}

There you have it. That's C# version rather than VB, but I think the general concepts should translate fairly readily. I hope that helps!

回答2:

This MSDN code sample illustrates how to iterate over a class's properties using reflection:

http://msdn.microsoft.com/en-us/library/kyaxdd3x.aspx#Y900

回答3:

Create a VB.Net Console application, copy and paste this code into the Module1.vb file and run it.

Module Module1

    Sub Main()
        For Each prop In GetType(TestClass).GetProperties()
            Console.WriteLine(prop.Name)
        Next

        Console.ReadKey(True)
    End Sub

End Module

Public Class TestClass
    Private _One As String = "1"
    Public Property One() As String
        Get
            Return _One
        End Get
        Set(ByVal value As String)
            _One = value
        End Set
    End Property

    Private _Two As Integer = 2
    Public Property Two() As Integer
        Get
            Return _Two
        End Get
        Set(ByVal value As Integer)
            _Two = value
        End Set
    End Property

    Private _Three As Double = 3.1415927
    Public Property Three() As Double
        Get
            Return _Three
        End Get
        Set(ByVal value As Double)
            _Three = value
        End Set
    End Property

    Private _Four As Decimal = 4.4D
    Public Property Four() As Decimal
        Get
            Return _Four
        End Get
        Set(ByVal value As Decimal)
            _Four = value
        End Set
    End Property
End Class