Maps ModelState values onto a property indicated by a lambda. Used to merge ModelStates from one model to another (i.e. PostModel to RenderModel). Usage: `ModelState.Apply(otherModel, m => m.SomeProperty)`

ModelStateDictionaryExtensions.cs

public static class ModelStateDictionaryExtensions
{
  public static TValue Apply<TSource, TValue>(this ModelStateDictionary modelState, TSource source, Expression<Func<TSource, TValue>> selectorExpression)
  {
    // Extract property info from the selector expression
    var memberExpression = selectorExpression.Body as MemberExpression;
    if(memberExpression == null)
      throw new ArgumentException($"Expression '{selectorExpression}' does not refer to a property", nameof(selectorExpression));

    var propertyInfo = memberExpression.Member as PropertyInfo;
    if(propertyInfo == null)
      throw new ArgumentException($"Expression '{selectorExpression}' does not refer to a property", nameof(selectorExpression));

    // Check if the model state contains the property
    var sourceValueAccessor = selectorExpression.Compile();
    if(!modelState.ContainsKey(propertyInfo.Name))
      return sourceValueAccessor(source);

    // Attempt to convert the model state value to the property type
    try
    {
      return (TValue)modelState[propertyInfo.Name].Value.ConvertTo(typeof(TValue));
    }
    catch(InvalidOperationException)
    {
      return sourceValueAccessor(source);
    }
  }
}