How to Dynamically Create Method from String for Lambda Expression

问题

My ultimate goal is to create a function that will dynamically pass method names to classes in the Hangfire library.

For example, here is the non-dynamic code which works:

RecurringJob.AddOrUpdate(() => myFunction(), Cron.Hourly)

The type of the first argument for AddOrUpdate is Expression<Action>. My first step was to use reflection to dynamically insert the function name:

Type thisControllerType = this.GetType();
MethodInfo method = thisControllerType.GetMethod(methodName); //methodName passed as string parameter
RecurringJob.AddOrUpdate(() => method.Invoke(this, null), Cron.Hourly);

When I check the Hangfire dashboard, it seems that this expression is being evaluated as MethodBase.Invoke. So I need help passing in the method name dynamically.

That may be enough info to answer my question, but another path I have taken is trying to generate the entire expression for the argument.

RecurringJob.AddOrUpdate(CreateCallExpression(method), Cron.Hourly);

public Expression<Action> CreateCallExpression(MethodInfo method)
    {
        //trying to pass in zero argument parameter, not sure if this syntax is correct    
        var parameter = System.Linq.Expressions.Expression.Parameter(typeof (Array));
        return System.Linq.Expressions.Expression.Lambda<Action>(System.Linq.Expressions.Expression.Call(method, parameter));
    }

In this case I am getting the exception {"Static method requires null instance, non-static method requires non-null instance.\r\nParameter name: method"}. I am working on that, but not sure if this is the road I should be going down. I have been working on this all day, so I was hoping someone might be able to help me speed up my learning.

回答1:

Your second instance will work in creating a pointer to your specified method, but to solve your static issue you just need to modify the following line in one of 2 ways. First you can complete the static reference by declaring the method you seek to be static and modifying this line of code:

System.Linq.Expressions.Expression.Call(method, parameter);

You would have to provide a null parameter for the call method because if you are searching for a static method, then the compiler will know exactly what method signature you desire, because there will only exist 1. The line of code would be updated to:

System.Linq.Expressions.Expression.Call(null, method, parameter);

The second approach is to define the class or "instance" that correlates to the method so that the compiler knows what class to search against for the method signature. You would have to modify your code like this:

var myInstance = Expression.Parameter(typeof(MyClass), "inst");
System.Linq.Expressions.Expression.Call(myInstance, method, parameter)

I recommend looking at the documentation for Call so that you know exactly how the pointer is being created.

来源：https://stackoverflow.com/questions/26411278/how-to-dynamically-create-method-from-string-for-lambda-expression

标签

asp.net-mvc

reflection

lambda

hangfire