Dynamically Using Methods in ASP.NET - The Article
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Types
Later on I will go into this in greater detail, but this will be just a quick overview here.
You probably know that when you create a class, you are essentially creating a type. A lot of people underestimate the power of a type, you may have used GetType once in a while, but you may not know the underlying power of it. The Type class contains a heck of a lot of information about an type that you can access and then can lead on to some pretty interesting abilities.
System.Reflection
The System.Reflection namespace contains all of the tools needed to provide a view (in code) of any assembly in .NET. If you take a look at the Object Browser in VS.NET, that is the equivalent of using System.Reflection to find out about all of that information. Intellisense in VS.NET is also like using System.Reflection and Lutz Roeder's .NET Reflector uses System.Reflection to browse an assembly.
But why would you use System.Reflection?
Let's say that you were given an assembly and you have no idea what it contains or does, you could use reflection to find out what's in it and how it works.
Or, if you were developing a dynamic documentation system, then you could just place an assembly in the /bin directory and develop an application that used reflection to dynamically generate information and documentation about the assembly (much like the Comment Web Pages with XML Code Comments in VS.NET).
MethodInfo
MethodInfo is a class within System.Reflection that allows you to get information about specific methods (which I will be using in this article). For example, you can give it a method and you can get all sorts of properties and methods back which you can use to examine that method.
Types - GetType
In the following examples, I will be using a simple ASP.NET page with code-behind (because in code-behind the class is clearly defined for you).
GetType returns a Type class with information about a particular type. Examine the following code -
Public Class WebForm3
Inherits System.Web.UI.Page
Private Sub Page_Load(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles MyBase.Load
Dim mtype As System.Type = GetType(WebForm3)
...
mtype now contains information about WebForm3. And if it were to do something like -
Response.Write(mtype.Assembly)
Response.Write("<br>")
Response.Write(mtype.FullName)
Response.Write("<br>")
Response.Write("<table border=""1"">")
Dim member As System.Reflection.MemberInfo
For Each member In mtype.GetMembers
Response.Write("<tr><td>")
Response.Write(member.Name)
Response.Write("</td><td>")
Response.Write(member.MemberType.ToString())
Response.Write("</td></tr>")
Next
Response.Write("</table>")
Live Demo
You will get a table of every single member in the Type. See the live demo.
I probably jumped ahead a bit with the GetMembers call, but all you need to know about it is that it returns an array of MemberInfo objects that contain information about every member in the type. If you take a look at the demo, you will see a lot of methods and properties that you didn't know existed. This is because it not only takes WebForm3, but also -
- System.Web.UI.Page
- System.Web.UI.TemplateControl
- System.Web.UI.Control
- System.Object
Because that is how far back the inheritance goes on WebForm3.
MethodInfo
MethodInfo is a class that stores method information and the GetMethod function of System.Type returns a fully loaded MethodInfo class for you to use.
Public Class WebForm3
Inherits System.Web.UI.Page
Private Sub Page_Load(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles MyBase.Load
Dim mtype As Type = GetType(WebForm3)
Dim minfo As System.Reflection.MethodInfo = mtype.GetMethod("Return1")
End Sub
Public Function Return1(ByVal name As String) As String
Return "Hello " & name
End Function
End Class
This piece of code fills minfo with lots of information about the function Return1.
MethodInfo - Parameters
Take a look at the following code -
Response.Write("<table border=""1"">")
Dim parameter As System.Reflection.ParameterInfo
For Each parameter In minfo.GetParameters()
Response.Write("<tr><td>")
Response.Write(parameter.Name)
Response.Write("</td><td>")
Response.Write(parameter.ParameterType.ToString())
Response.Write("</td></tr>")
Next
Response.Write("</table>")
Live Demo
This does very much the same as the MemberInfo one before, but this time with parameters of a function. You will notice that it only returns one parameter (name) for Return1.
MethodInfo - Invoking
Now here is the really interesting part - You can also call a method using MethodInfo.
Let's say that you had the name of a method stored in a variable. You can't go - variablename(). So you have to use MethodInfo.
MethodInfo.Invoke takes two parameters - The instance that created the method and the parameters (in an object array).
Take a look at the following code -
Public Class WebForm3
Inherits System.Web.UI.Page
Private Sub Page_Load(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles MyBase.Load
Dim mtype As Type = GetType(WebForm3)
Dim minfo As System.Reflection.MethodInfo = mtype.GetMethod("Return1")
Dim myparamarray() As Object = {"Sample Name"} 'New
Response.Write(minfo.Invoke(Me, myparamarray)) 'New
End Sub
Public Function Return1(ByVal name As String) As String
Return "Hello " & name
End Function
End Class
The two new lines create a parameter array with a field for each parameter and it's value and then uses minfo.Invoke to call the function and write it's results to the screen. The same would work with a method.
Next: Summary >>
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