Use reflection to find a private field within `this` class, and instantiate it

Question

Hypothetical situation. Say I had a class which contained numerous private fields. I want it to instantiate every field that it can find with the correct type. So far, I have used

public class TestClass
{
    private SomeClass sc;
    private AnotherClass ac;

    public TestClass()
    {
        var type = GetType();
        var fields = type.GetFields(BindingFlags.NonPublic | BindingFlags.Instance)
            .Select(x => x.Name)
            .ToList();

        foreach (var f in fields)
            type.GetField(f).SetValue(/*instantiate here*/);
    }
}

How would one instantiate it? (this is assuming the new() constructor in each class is parameterless and is not empty)

Filter fields on clr type? Then use set value passing the desired object to apply it to — Mardoxx, Apr 07 '18 at 14:35
Instantiate it with what? Are you sure that they all have default `new()` constructors? What happens when they only have parameterized (or no public) constructors? — Ron Beyer, Apr 07 '18 at 14:44
@RonBeyer I will update my question to be more specific. Assume they all have non-default `new()` constructors which take no arguments. — Kontorted, Apr 07 '18 at 14:45
[`Activator.CreateInstance(Type)`](https://msdn.microsoft.com/en-us/library/wccyzw83(v=vs.110).aspx) — Ron Beyer, Apr 07 '18 at 14:46
so, like `var field = type.GetField(f);` `field.SetValue(field, Activator.CreateInstance(field.GetType()));` ? — Kontorted, Apr 07 '18 at 14:49
Yes, although the arguments for `SetValue` are `SetValue(object_to_set, value_to_set)`, so it should be `SetValue(this, Activator.CreateInstance(...))` — Ron Beyer, Apr 07 '18 at 14:51
@RonBeyer Would you like to create an answer post so that I can mark it as accepted? — Kontorted, Apr 07 '18 at 14:56

Ron Beyer · Accepted Answer · 2018-04-07T15:07:19.017

Activator.CreateInstance(Type) can create an instance from the type. This requires a parameter-less constructor (there are overloads for parameters).

To use it, just modify your code a little bit:

public class TestClass
{
    private SomeClass sc;
    private AnotherClass ac;

    public TestClass()
    {
        var type = GetType();

        type.GetFields(BindingFlags.NonPublic | BindingFlags.Instance)
            .ToList()
            .ForEach(f => {
                f.SetValue(this, Activator.CreateInstance(f.FieldType);
            });
    }
}

Use reflection to find a private field within `this` class, and instantiate it

1 Answers1