Reflection

  • Thread starter Thread starter BK
  • Start date Start date
B

BK

I've used reflection quite a bit to launch forms, it works great,
requires little coding, and allows me to launch forms from a dynamic
menu.

Now I have a need to instantiate any one of several business classes
dynamically, so my natural inclination was to use reflection. The
problem I'm running into is that my business classes require arguments
to be passed in where as the forms did not. Here is an example of
launching a form:

Dim ExternalAssembly As System.Reflection.Assembly = _
System.Reflection.Assembly.LoadFrom(Application.StartupPath &
"\" & Container)
Dim CalledForm As BravoBaseForm = _
ExternalAssembly.CreateInstance(FullName, True)
CalledForm.Show()

Container, in my example, is an assembly containing the form,
typically an Exe or a Dll. Fullname refers to the class name
representing the form I want to launch. This code works fine for
launching a form. Now contrast that with the following code:

ExternalAssembly = System.Reflection.Assembly.LoadFrom(TargetHost)
BizClass = Me.ExternalAssembly.CreateInstance(ClassName, _
True, BindingFlags.CreateInstance, Nothing, args, _
System.Globalization.CultureInfo.CurrentCulture, Nothing)

To pass in arguments, I have to use another overloaded CreateInstance
call. I'll be honest, I'm not sure what all of the arguments are, I'm
making somewhat educated guesses on them. The main ones I do
understand is the first argument which is the name of the class I want
to instantiate, and the fifth one which is an array of objects
representing the parameters required to instantiate the class.

The code in my second example executes without any errors, but
BizClass is equal to nothing afterwards. Any thoughts or comments are
appreciated.
 
classes are a waste of time; they slow your programs down

80% of vb6 developers didn't need COM components
 
I've used reflection quite a bit to launch forms, it works great,
requires little coding, and allows me to launch forms from a dynamic
menu.

Now I have a need to instantiate any one of several business classes
dynamically, so my natural inclination was to use reflection. The
problem I'm running into is that my business classes require arguments
to be passed in where as the forms did not. Here is an example of
launching a form:

Dim ExternalAssembly As System.Reflection.Assembly = _
System.Reflection.Assembly.LoadFrom(Application.StartupPath &
"\" & Container)
Dim CalledForm As BravoBaseForm = _
ExternalAssembly.CreateInstance(FullName, True)
CalledForm.Show()

Container, in my example, is an assembly containing the form,
typically an Exe or a Dll. Fullname refers to the class name
representing the form I want to launch. This code works fine for
launching a form. Now contrast that with the following code:

ExternalAssembly = System.Reflection.Assembly.LoadFrom(TargetHost)
BizClass = Me.ExternalAssembly.CreateInstance(ClassName, _
True, BindingFlags.CreateInstance, Nothing, args, _
System.Globalization.CultureInfo.CurrentCulture, Nothing)

To pass in arguments, I have to use another overloaded CreateInstance
call. I'll be honest, I'm not sure what all of the arguments are, I'm
making somewhat educated guesses on them. The main ones I do
understand is the first argument which is the name of the class I want
to instantiate, and the fifth one which is an array of objects
representing the parameters required to instantiate the class.

The code in my second example executes without any errors, but
BizClass is equal to nothing afterwards. Any thoughts or comments are
appreciated.
Click to expand...

Hi,

Take a look at what I just posted in the following thread. I
presented two methods for creating a String object via reflection.
Each method required passing parameters to the constructor.

http://groups.google.com/group/micr...dcd7f2aec7c/37cc4c00915f1fd5#37cc4c00915f1fd5

Brian
 
Hi,

Take a look at what I just posted in the following thread. I
presented two methods for creating a String object via reflection.
Each method required passing parameters to the constructor.

http://groups.google.com/group/microsoft.public.dotnet.languages.vb/b...

Brian
Click to expand...

Thanks for the reply, however neither of your examples will work. The
line:

Dim typ As Type = GetType(String)

wouldn't work for me since my type is unknown at design time. Think
of it this way. Lets say I have 5 assemblies, all dll's, that hold
some unknown number of classes in them. Lets also assume for
simplicity sake that all of the classes in any of the dlls conform to
a known interface called iBusinessObject. Programmatically, I want to
instantiate any one of these classes but I won't know ahead of time
which one. Lets also say that the name of the class is known, but it
is a string variable. How would I get the type so I can instantiate
it?
 
Thanks for the reply, however neither of your examples will work. The
line:

Dim typ As Type = GetType(String)

wouldn't work for me since my type is unknown at design time. Think
of it this way. Lets say I have 5 assemblies, all dll's, that hold
some unknown number of classes in them. Lets also assume for
simplicity sake that all of the classes in any of the dlls conform to
a known interface called iBusinessObject. Programmatically, I want to
instantiate any one of these classes but I won't know ahead of time
which one. Lets also say that the name of the class is known, but it
is a string variable. How would I get the type so I can instantiate
it?
Click to expand...

Hi,

No problem. You can do this instead to load a type at runtime.

Dim typ as Type = Type.GetType("YourNamespace.YourType")

Or if the assembly your type is in isn't loaded yet do this.

Dim asm As Assembly = Assembly.Load("YourAssembly")
Dim typ As Type = asm.GetType("YourNamespace.YourType")

Brian
 
Thanks, but now I try this:

Dim typ As Type = ExternalAssembly.GetType(FullClassName)
Me.BizClass = DirectCast(Me.BizClass, typ)

and I get an error telling me "Type typ is not defined". Mind you if
I comment the second line of code out, it runs just fine.

?????
 
When you use DirectCast the second parameter is the object type, not an
instance Type. If you are trying to cast to various type names you may have
to come up with a better way since this way is for specific types. I use
DirectCast only when I know what type to cast to. I don't think it works
dynamically.

Lloyd Sheen
 
BK said:
Thanks, but now I try this:

Dim typ As Type = ExternalAssembly.GetType(FullClassName)
Me.BizClass = DirectCast(Me.BizClass, typ)

and I get an error telling me "Type typ is not defined". Mind you
if I comment the second line of code out, it runs just fine.

?????
Click to expand...

Directcast requires a type name, not a type object. What you're trying is
contradictive:

If you specify a variable type, you already know the type of object that you
want to assign to the variable. Consequently you can write the same type
name when you use Directcast.

If you have a System.Type object, you do not know the type of the variable
when writing the code. As a consequence, you can not know how to declare the
variable - well, it's always "Object" at least.


Armin
 
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