Invoking Methods with Optional Parameters Through Reflection

Invoking methods with optional parameters through reflection

According to MSDN, to use the default parameter you should pass Type.Missing.

If your constructor has three optional arguments then instead of passing an empty object array you'd pass a three element object array where each element's value is Type.Missing, e.g.

type.GetParameterlessConstructor()
    .Invoke(BindingFlags.OptionalParamBinding | 
            BindingFlags.InvokeMethod | 
            BindingFlags.CreateInstance, 
            null, 
            new object[] { Type.Missing, Type.Missing, Type.Missing }, 
            CultureInfo.InvariantCulture);

How to call Method with optional parameters using reflections c#

Optional parameters: aren't actually optional - all that happens is that the compiler normally supplies the omitted value automatically for you. Since you're not using a compiler here, you'll need to supply it yourself, using new object[] {request, constants, null}. Note that if you want to properly respect the default value (rather than knowing it is null in this case), you'd need to look at the ParameterInfo, specifically .HasDefaultValue and .DefaultValue.

Example (not using ParameterInfo, note):

using System;
using System.Reflection;

class P
{
    static void Main()
    {
        string request = "r", constants = "c", count = "#";
        var classInstance = new P();

        typeof(P).GetMethod(nameof(Method1),
            BindingFlags.Instance | BindingFlags.NonPublic)
            .Invoke(classInstance, new object[] { request, constants, null });

        typeof(P).GetMethod(nameof(Method1),
            BindingFlags.Instance | BindingFlags.NonPublic)
            .Invoke(classInstance, new object[] { request, constants, count });

        typeof(P).GetMethod(nameof(Method2),
            BindingFlags.Instance | BindingFlags.NonPublic)
            .Invoke(classInstance, new object[] { request, constants });
    }

    void Method1(string request, string constants, string count = null)
        => Console.WriteLine($"#1: {request}, {constants}, {count}");

    void Method2(string request, string constants)
        => Console.WriteLine($"#2: {request}, {constants}");
}

Invoke method with optional arguments

Try with:

methodInfo.Invoke(this, new object[] { args });

The parameter is only 1 and is an array of object. If you pass args it consider multiple parameters.

How invoke method for a method by default value for parameters by reflection

You can use ParameterInfo.HasDefaultValue and ParameterInfo.DefaultValue to detect this. You'd need to check whether the number of arguments you've been given is equal to the number of parameters in the method, and then find the ones with default values and extract those default values.

For example:

var parameters = method.GetParameters();
object[] args = new object[parameters.Length];
for (int i = 0; i < args.Length; i++)
{
    if (i < providedArgs.Length)
    {
        args[i] = providedArgs[i];
    }
    else if (parameters[i].HasDefaultValue)
    {
        args[i] = parameters[i].DefaultValue;
    }
    else
    {
        throw new ArgumentException("Not enough arguments provided");
    }
}
method.Invoke(method.IsStatic ? null : this, args);

How to call the method with optional arguments dynamically?

You can do it using reflection:

• Obtain MethodInfo passing all three parameter types.
• Obtain run-time parameter values
• Obtain parameter metadata ParameterInfo\[\] from MethodInfo by calling GetParameters()
• For each missing parameter, check HasDefaultValue, and grab DefaultValue if it does
• Append an array of default values to the array of values passed in. You will have an array of three objects
• Pass the resultant array to the method that you obtained using reflection.

Reflection: How to Invoke Method with parameters

Change "methodInfo" to "classInstance", just like in the call with the null parameter array.

  result = methodInfo.Invoke(classInstance, parametersArray);

Invoke Method with reflection pass through parameters and receive a return value

Have a look at the signature of MethodInfo.Invoke

public object Invoke(
    object obj,
    object[] parameters
)

You need to pass the parameters of the method in the parameters object[]. Note that the method returns an object, you just need to cast it to the result type.

A call to TestString should look like this:

var parameters = new object[]{"A string", 10, 'a'};
string result = (string) t.GetMethod("Print").Invoke(t, parameters);

Java reflection API: Invoking a method without parameters

No exception is thrown but myBytes stays null

Correct, what you wanted there was:

byte[] myBytes = (byte[])m.invoke(yourInstance);

More in the documentation. Notes:

• The return value of the method is the return value of invoke.
• The first argument to invoke is the instance on which to call the method (since you've listed an instance method, not a static method; if it were static, the first argument would be null). You haven't shown a variable referring to the instance anywhere, so I called it yourInstance in the above.

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