Get PropertyInfo of a parameter passed as lambda expression

Question

For example, I have a class:

public class Person
{
  public int Id;
  public string Name, Address;
}

and I want to call a method to update info in this class base on Id:

update(myId, myPerson => myPerson.Name = "abc");

explain: This method will query from database and gets Person entity given a myId, then it sets Name to "abc", so it does the same job as I call this:

update(myId, myPerson => myPerson.Address = "my address");

is it possible? If so how?

Answer 1

I wouldn't use PropertyInfo, just like Reed Copsey said in his answer, but just for information, you can extract the PropertyInfo of an expression with this:

public PropertyInfo GetPropertyFromExpression<T>(Expression<Func<T, object>> GetPropertyLambda)
{
    MemberExpression Exp = null;

    //this line is necessary, because sometimes the expression comes in as Convert(originalexpression)
    if (GetPropertyLambda.Body is UnaryExpression)
    {
        var UnExp = (UnaryExpression)GetPropertyLambda.Body;
        if (UnExp.Operand is MemberExpression)
        {
            Exp = (MemberExpression)UnExp.Operand;
        }
        else
            throw new ArgumentException();
    }
    else if (GetPropertyLambda.Body is MemberExpression)
    {
        Exp = (MemberExpression)GetPropertyLambda.Body;
    }
    else
    {
        throw new ArgumentException();
    }

    return (PropertyInfo)Exp.Member;
}

---

In the case of a composite expression like MyPerson.PersonData.PersonID, you could go getting the sub expressions until they're not MemberExpressions anymore.

public PropertyInfo GetPropertyFromExpression<T>(Expression<Func<T, object>> GetPropertyLambda)
{
    //same body of above method without the return line.
    //....
    //....
    //....

    var Result = (PropertyInfo)Exp.Member;

    var Sub = Exp.Expression;

    while (Sub is MemberExpression)
    {
        Exp = (MemberExpression)Sub;
        Result = (PropertyInfo)Exp.Member;
        Sub = Exp.Expression;
    }

    return Result;
    //beware, this will return the last property in the expression.
    //when using GetValue and SetValue, the object needed will not be
    //the first object in the expression, but the one prior to the last.
    //To use those methods with the first object, you will need to keep
    //track of all properties in all member expressions above and do
    //some recursive Get/Set following the sequence of the expression.
}

Answer 2

Even cleaner IMHO (another variant on @DanielMöller's post)

        /// <summary>
        ///     Gets the corresponding <see cref="PropertyInfo" /> from an <see cref="Expression" />.
        /// </summary>
        /// <param name="expression">The expression that selects the property to get info on.</param>
        /// <returns>The property info collected from the expression.</returns>
        /// <exception cref="ArgumentNullException">When <paramref name="expression" /> is <c>null</c>.</exception>
        /// <exception cref="ArgumentException">The expression doesn't indicate a valid property."</exception>
        public static PropertyInfo GetPropertyInfo<T>(this Expression<Func<T, object>> expression)
        {
            switch (expression?.Body) {
                case null:
                    throw new ArgumentNullException(nameof(expression));
                case UnaryExpression unaryExp when unaryExp.Operand is MemberExpression memberExp:
                    return (PropertyInfo)memberExp.Member;
                case MemberExpression memberExp:
                    return (PropertyInfo)memberExp.Member;
                default:
                    throw new ArgumentException($"The expression doesn't indicate a valid property. [ {expression} ]");
            }
        }

Answer 3

This is possible, and there's no need to use PropertyInfo.

You'd design your method like so:

public bool Update<T>(int id, Action<T> updateMethod)
    // where T  : SomeDbEntityType
{
    T entity = LoadFromDatabase(id); // Load your "person" or whatever

    if (entity == null) 
        return false; // If you want to support fails this way, etc...

    // Calls the method on the person
    updateMethod(entity);

    SaveEntity(entity); // Do whatever you need to persist the values

    return true;
}

Answer 4

Here is a version of @DanielMöller's answer updated for modern syntax, with specified exception messages, and documentation.

/// <summary>
///     Gets the corresponding <see cref="PropertyInfo" /> from an <see cref="Expression" />.
/// </summary>
/// <param name="property">The expression that selects the property to get info on.</param>
/// <returns>The property info collected from the expression.</returns>
/// <exception cref="ArgumentNullException">When <paramref name="property" /> is <c>null</c>.</exception>
/// <exception cref="ArgumentException">The expression doesn't indicate a valid property."</exception>
private PropertyInfo GetPropertyInfo<T, P>(Expression<Func<T, P>> property)
{
    if (property == null) {
        throw new ArgumentNullException(nameof(property));
    }

    if (property.Body is UnaryExpression unaryExp) {
        if (unaryExp.Operand is MemberExpression memberExp) {
            return (PropertyInfo)memberExp.Member;
        }
    }
    else if (property.Body is MemberExpression memberExp) {
        return (PropertyInfo)memberExp.Member;
    }

    throw new ArgumentException($"The expression doesn't indicate a valid property. [ {property} ]");
}