PropertyInfo SetValue and nulls

Question

If I have something like:

object value = null;
Foo foo = new Foo();

PropertyInfo property = Foo.GetProperties().Single(p => p.Name == "IntProperty");
property.SetValue(foo, value, null);

Then foo.IntProperty gets set to 0, even though value = null. It appears it's doing something like IntProperty = default(typeof(int)). I would like to throw an InvalidCastException if IntProperty is not a "nullable" type (Nullable<> or reference). I'm using Reflection, so I don't know the type ahead of time. How would I go about doing this?

Solution 1

If you have the PropertyInfo, you can check the .PropertyType; if .IsValueType is true, and if Nullable.GetUnderlyingType(property.PropertyType) is null, then it is a non-nullable value-type:

        if (value == null && property.PropertyType.IsValueType &&
            Nullable.GetUnderlyingType(property.PropertyType) == null)
        {
            throw new InvalidCastException ();
        }

Solution 2

You can use PropertyInfo.PropertyType.IsAssignableFrom(value.GetType()) expression to determine whether specified value can be written into property. But you need to handle case when value is null, so in this case you can assign it to property only if property type is nullable or property type is reference type:

public bool CanAssignValueToProperty(PropertyInfo propertyInfo, object value)
{
    if (value == null)
        return Nullable.GetUnderlyingType(propertyInfo.PropertyType) != null ||
               !propertyInfo.IsValueType;
    else
        return propertyInfo.PropertyType.IsAssignableFrom(value.GetType());
}

Also, you may find useful Convert.ChangeType method to write convertible values to property.

Comments

[Nelson Rothermel]

That's it. I was messing with .PropertyType.IsClass, but wasn't getting very far.

[Nelson Rothermel]

SetValue() already throws an exception when it can't set the value, which is desired behavior (but it's an ArgumentException). I only need to handle the null scenario.