Invoke a class from a string

[Simon King]

I have created a service which uses the FileSystemWatcher to monitor folders
for file changes. I have a app.config file which specifies which folders to
watch and what to do when a change occurs.

What I would like to do is specify a class and a method which can be invoked
when that file change occurs. I can specify the name of the class/method in
the config file but the app will see it as a string.

Therefore my question is, can I invoke a class and method from a string
value. If so how would I do that.

Regards
Simon

[Jon Skeet [C# MVP]]

<"=?Utf-8?B?U2ltb24gS2luZw==?=" <Simon
Ki**@discussions.microsoft.com>> wrote:

I have created a service which uses the FileSystemWatcher to monitor folders
for file changes. I have a app.config file which specifies which folders to
watch and what to do when a change occurs.

What I would like to do is specify a class and a method which can be invoked
when that file change occurs. I can specify the name of the class/method in
the config file but the app will see it as a string.

Therefore my question is, can I invoke a class and method from a string
value. If so how would I do that.

Is the method a static method? If so, just use Type.GetType with the
class name (or Assembly.GetType if it'll be in a different assembly
from the calling one) to get the type, and Type.GetMethod to get the
method itself. You can then call Invoke on the MethodInfo, passing null
as the target argument. If it's an instance method, you'll need to
create an instance of the class first, which you can do with
Activator.CreateInstance.

--
Jon Skeet - <sk***@pobox.com>
http://www.pobox.com/~skeet
If replying to the group, please do not mail me too

[Frans Bouma [C# MVP]]

Simon King wrote:

I have created a service which uses the FileSystemWatcher to monitor
folders for file changes. I have a app.config file which specifies which
folders to watch and what to do when a change occurs.

What I would like to do is specify a class and a method which can be
invoked when that file change occurs. I can specify the name of the
class/method in the config file but the app will see it as a string.

Therefore my question is, can I invoke a class and method from a string
value. If so how would I do that.

Sure :)
For complete control, you'll need the assembly name and the type name of the
class and the method to call. Your code which will call the class has to be
aware at compile time of the type of the class (so implement an interface) or
if that's not the case you need to use reflection to grab the methodinfo
object to invoke the method, which is slower.

Here's an example of a routine which creates an instance of an object in a
particular assembly:

public static IDBDriver CreateDBDriverInstance(string assemblyFilename,
string namespaceToUse, string className)
{
// load the assembly
Assembly databaseDriver = Assembly.LoadFrom(assemblyFilename);

string completeClassPath = className;
if(namespaceToUse.Length > 0)
{
// there is a namespace specified, add it
completeClassPath = namespaceToUse + "." + className;
}

// create the instance
IDBDriver toReturn =
(IDBDriver)databaseDriver.CreateInstance(completeC lassPath);

// return the instance
return toReturn;
}

The instance can then be invoked by calling the method directly or if you
want to invoke a method you define in the config, you'll need reflection:

IDBDriver driver =
Utils.CreateDBDriverInstance("SD.LLBLGen.Pro.DBDri vers.SqlServer.dll",
"SD.LLBLGen.Pro.DBDrivers.SqlServer", "SqlServerDBDriver");

I can now call the method if I know what to call:
driver.SomeMethod();

if I don't know what to call at compile time, because it is read from the
config file, follow this:

// get method info of method without any parameters. methodToCall is a string
// with the method name read from the config file.
MethodInfo myMethod = driver.GetType().GetMethod(mthodToCall);
// call it
myMethod.Invoke(driver, null);

Frans.

--
Get LLBLGen Pro, productive O/R mapping for .NET: http://www.llblgen.com
My .NET Blog: http://weblogs.asp.net/fbouma
Microsoft C# MVP

[Simon King]

Fans and Jon,

Thats exactly what I need fantastic thanks. I'll let you know how I get on.

Regards
Simon

"Frans Bouma [C# MVP]" wrote:

Simon King wrote:

I have created a service which uses the FileSystemWatcher to monitor
folders for file changes. I have a app.config file which specifies which
folders to watch and what to do when a change occurs.

What I would like to do is specify a class and a method which can be
invoked when that file change occurs. I can specify the name of the
class/method in the config file but the app will see it as a string.

Therefore my question is, can I invoke a class and method from a string
value. If so how would I do that.

Sure :)
For complete control, you'll need the assembly name and the type name of the
class and the method to call. Your code which will call the class has to be
aware at compile time of the type of the class (so implement an interface) or
if that's not the case you need to use reflection to grab the methodinfo
object to invoke the method, which is slower.

Here's an example of a routine which creates an instance of an object in a
particular assembly:

public static IDBDriver CreateDBDriverInstance(string assemblyFilename,
string namespaceToUse, string className)
{
// load the assembly
Assembly databaseDriver = Assembly.LoadFrom(assemblyFilename);

string completeClassPath = className;
if(namespaceToUse.Length > 0)
{
// there is a namespace specified, add it
completeClassPath = namespaceToUse + "." + className;
}

// create the instance
IDBDriver toReturn =
(IDBDriver)databaseDriver.CreateInstance(completeC lassPath);

// return the instance
return toReturn;
}

The instance can then be invoked by calling the method directly or if you
want to invoke a method you define in the config, you'll need reflection:

IDBDriver driver =
Utils.CreateDBDriverInstance("SD.LLBLGen.Pro.DBDri vers.SqlServer.dll",
"SD.LLBLGen.Pro.DBDrivers.SqlServer", "SqlServerDBDriver");

I can now call the method if I know what to call:
driver.SomeMethod();

if I don't know what to call at compile time, because it is read from the
config file, follow this:

// get method info of method without any parameters. methodToCall is a string
// with the method name read from the config file.
MethodInfo myMethod = driver.GetType().GetMethod(mthodToCall);
// call it
myMethod.Invoke(driver, null);

Frans.

--
Get LLBLGen Pro, productive O/R mapping for .NET: http://www.llblgen.com
My .NET Blog: http://weblogs.asp.net/fbouma
Microsoft C# MVP