Entity Framework will only set related entity property to “null” if I first get the property

Question

Edit This seems to occur for any Entity property that references another entity in one direction. In other words, for the below example, the fact

Answer 1

Personally, I think Nathan's answer (lazy-loading inside the property setter) is the most robust. However, it mushrooms your domain classes (10 lines per property) and makes it less readable.

As another workaround, I compiled two methods into a extension method:

public static void SetToNull(this TEntity entity, Expression> navigationProperty, DbContext context = null)
    where TEntity : class
    where TProperty : class
{
    var pi = GetPropertyInfo(entity, navigationProperty);

    if (context != null)
    {
        //If DB Context is supplied, use Entry/Reference method to null out current value
        context.Entry(entity).Reference(navigationProperty).CurrentValue = null;
    }
    else
    {
        //If no DB Context, then lazy load first
        var prevValue = (TProperty)pi.GetValue(entity);
    }

    pi.SetValue(entity, null);
}

static PropertyInfo GetPropertyInfo(    TSource source,    Expression> propertyLambda)
{
    Type type = typeof(TSource);

    MemberExpression member = propertyLambda.Body as MemberExpression;
    if (member == null)
        throw new ArgumentException(string.Format(
            "Expression '{0}' refers to a method, not a property.",
            propertyLambda.ToString()));

    PropertyInfo propInfo = member.Member as PropertyInfo;
    if (propInfo == null)
        throw new ArgumentException(string.Format(
            "Expression '{0}' refers to a field, not a property.",
            propertyLambda.ToString()));

    if (type != propInfo.ReflectedType &&
        !type.IsSubclassOf(propInfo.ReflectedType))
        throw new ArgumentException(string.Format(
            "Expression '{0}' refers to a property that is not from type {1}.",
            propertyLambda.ToString(),
            type));

    return propInfo;
}

This allows you to supply a DbContext if you have one, in which case it will use the most efficient method and set the CurrentValue of the Entry Reference to null.

entity.SetToNull(e => e.ReferenceProperty, dbContext);

If no DBContext is supplied, it will lazy load first.

entity.SetToNull(e => e.ReferenceProperty);