C# Why dictionary allows property setting though the property is internal set

Question

I'm using fluent validation API for writing validations.

I came across a internal set property on ValidationContext class.

public class ValidationContext {
        public Dictionary<string, object> RootContextData { get; internal set; } = new Dictionary<string, object>();
}

I came across the internal set property in this question. So, I can set the value of a dictionary like this

var rowItemContext = new ValidationContext();
rowItemContext.RootContextData.Add("ab", new object());//this works

The above code to add an item to the RootcontextData works.

But the following line to directly assign another dictionary to RootContextData results in an compile time error.

Property or indexer 'ValidationContext.RootContextData' cannot be assigned to -- it is read only

rowItemContext.RootContextData = new Dictionary<string, object>()
            {
                { "ab", new object() }
            };

So, to add a dictionary of items to context data I have to loop over the dictionary to add. I'm surprised to see why the first one works but not the direct assignment.

Answer 1

It seems like that you got confused as to what counts as "setting" a property.

Generally, you are only setting the property when you use the = sign. When you use the assignment operator, you are actually giving the property a new value.

Dictionary is a reference type, so the property stores a reference to the dictionary object. The value of the property in this case, is the reference pointing to the dictionary.

When you call Add, you are just modifying the dictionary object, not the property's value. The property's value still points to the same dictionary object so it remains unchanged.

When you use = and creates a new dictionary, you are trying to change the reference stored in the property to some other reference. This counts as setting a property, so you can't do it.

Answer 2

There are several moments:

1. You use internal set, so you can set property only inside your assembly. see also: What is internal set property in c#?
2. In the second case you use Collection Initializer. It sets property with new value (new dictionary initialized with key "ab" and value new object())
3. If you get such exception that you try to do this outside your assembly. Use Add method.
4. From design perspective: don't use public properties that can mutate internal state of your object. Incapsulate it.