CODEWITHSUNDEEP
c#.netreflectionattributesdefault-value
greggorob64
greggorob64

Reputation: 2557

Get the default value of a property given its name

I have a property, (example shown below).

 [DefaultValue(false)]
 public bool MyProperty {
    get {
       return myVal;
    }
    set {
       myVal=value;
    }
 }

The situation I'm using this is to make sure it shows up as bold in the a PropertyGrid if the default value is not set.

I find it incredibly annoying that in my constructor, I have to set the initial value of my property, and hope that they match up.

Is it possible to have my constructor "discover" the default value of a given property, and set it accordingly? Something like:

myctor()
{
   myVal = GetDefaultValueProperty<bool>("MyProperty");
}

Upvotes: 6

Views: 6401

Answers (3)

Rudey
Rudey

Reputation: 4975

Here's a generic extension method based on Paul Turner's answer. It will work for any member of any class.

public static bool TryGetDefaultValue<TSource, TResult>(this TSource _, Expression<Func<TSource, TResult>> expression, out TResult result)
{
    if (((MemberExpression)expression.Body).Member.GetCustomAttribute(typeof(DefaultValueAttribute)) is DefaultValueAttribute attribute)
    {
        result = (TResult)attribute.Value;
        return true;
    }

    result = default;
    return false;
}

Alternatively, you're OK with the default value being returned if the attribute was not found, use this:

public static TResult GetDefaultValue<TSource, TResult>(this TSource _, Expression<Func<TSource, TResult>> expression)
{
    if (((MemberExpression)expression.Body).Member.GetCustomAttribute(typeof(DefaultValueAttribute)) is DefaultValueAttribute attribute)
    {
        return (TResult)attribute.Value;
    }

    return default;
}

You could also modify it to throw an exception if the attribute wasn't found.

Upvotes: 0

Paul Turner
Paul Turner

Reputation: 39625

You can use the following code to get the metadata you're after.

public static T GetDefaultValue<T>(string propertyName)
{
    var property = typeof(MyClass).GetProperty(propertyName);

    var attribute = property
        .GetCustomAttribute(typeof(DefaultValueAttribute)) 
            as DefaultValueAttribute;

    if(attribute != null)
    {
        return (T)attribute.Value;
    }
}

If you want to do something really cool, you can do this with a Lambda expression:

public static T GetDefaultValue<T>(
    Expression<Func<T, MyClass>> propertySelector)
{
    MemberExpression memberExpression = null;

    switch (expression.Body.NodeType)
    {
        case ExpressionType.MemberAccess:
            // This is the default case where the 
            // expression is simply member access.
            memberExpression 
                = expression.Body as MemberExpression;
            break;

        case ExpressionType.Convert:
            // This case deals with conversions that 
            // may have occured due to typing.
            UnaryExpression unaryExpression 
                = expression.Body as UnaryExpression;

            if (unaryExpression != null)
            {
                memberExpression 
                    = unaryExpression.Operand as MemberExpression;
            }
            break;
    }


    MemberInfo member = memberExpression.Member;

    // Check for field and property types. 
    // All other types are not supported by attribute model.
    switch (member.MemberType)
    {
        case MemberTypes.Property:
            break;
        default:
            throw new Exception("Member is not property");
    }

    var property = (PropertyInfo)member;

    var attribute = property
        .GetCustomAttribute(typeof(DefaultValueAttribute)) 
            as DefaultValueAttribute;

    if(attribute != null)
    {
        return (T)attribute.Value;
    }
}

Usage is then:

myctor()
{
   myVal = GetDefaultValue(x => x.MyProperty);
}

Upvotes: 9

SLaks
SLaks

Reputation: 887453

You can call the GetProperty method to find the property, then call GetCustomAttributes(typeof(DefaultValueAttribute) (and cast its result) to get the attribute applied.

Upvotes: 6

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