ChatGPT解决这个技术问题 Extra ChatGPT

How to detect if a property exists on an ExpandoObject?

In javascript you can detect if a property is defined by using the undefined keyword:

if( typeof data.myProperty == "undefined" ) ...

How would you do this in C# using the dynamic keyword with an ExpandoObject and without throwing an exception?

@CodeInChaos: Note that the displayed code doesn't check the value of data.myProperty; it checks what typeof data.myProperty returns. It is correct that data.myProperty may exist and be set to undefined, but in that case, typeof will return something other than "undefined". So this code does work.

D
Dykam

According to MSDN the declaration shows it is implementing IDictionary:

public sealed class ExpandoObject : IDynamicMetaObjectProvider, 
    IDictionary<string, Object>, ICollection<KeyValuePair<string, Object>>, 
    IEnumerable<KeyValuePair<string, Object>>, IEnumerable, INotifyPropertyChanged

You can use this to see if a member is defined:

var expandoObject = ...;
if(((IDictionary<String, object>)expandoObject).ContainsKey("SomeMember")) {
    // expandoObject.SomeMember exists.
}

To make this check simpler, I've overloaded TryGetValue and make it always return true, setting the return value to "undefined" if the property was not defined. if( someObject.someParam != "undefined" ) ... And it works :)
Also possible :), but I think you meant "overridden" instead of overloaded.
yep. I've again changed "undefined" to a special object's const value i created elsewhere. It prevents casting problems :p
I believe this solution is still current; don't take anyone's word for the price of reflection -- test it for yourself and see if you can afford it
@BlueRaja-DannyPflughoeft Yes, it is. Without the cast it will just be a dynamic invocation, with the case you get in the internals. More specifically, it is explicitly implemented: github.com/mono/mono/blob/master/mcs/class/dlr/Runtime/…
C
Chris Moschini

An important distinction needs to be made here.

Most of the answers here are specific to the ExpandoObject which is mentioned in the question. But a common usage (and reason to land on this question when searching) is when using the ASP.Net MVC ViewBag. That's a custom implementation/subclass of DynamicObject, which won't throw an Exception when you check any arbitrary property name for null. Suppose you might declare a property like:

@{
    ViewBag.EnableThinger = true;
}

Then suppose you wanted to check its value, and whether it's even set - whether it exists. The following is valid, will compile, won't throw any exceptions, and gives you the right answer:

if (ViewBag.EnableThinger != null && ViewBag.EnableThinger)
{
    // Do some stuff when EnableThinger is true
}

Now get rid of the declaration of EnableThinger. Same code compiles and runs properly. No need for reflection.

Unlike ViewBag, ExpandoObject will throw if you check for null on a property that doesn't exist. In order to get MVC ViewBag's gentler functionality out of your dynamic objects, you'll need to use an implementation of dynamic that doesn't throw.

You could simply use the exact implementation in MVC ViewBag:

. . .
public override bool TryGetMember(GetMemberBinder binder, out object result)
{
    result = ViewData[binder.Name];
    // since ViewDataDictionary always returns a result even if the key does not exist, always return true
    return true;
}
. . .

https://github.com/ASP-NET-MVC/aspnetwebstack/blob/master/src/System.Web.Mvc/DynamicViewDataDictionary.cs

You can see it being tied into MVC Views here, in MVC ViewPage:

http://aspnetwebstack.codeplex.com/SourceControl/latest#src/System.Web.Mvc/ViewPage.cs

The key to DynamicViewDataDictionary's graceful behavior is the Dictionary implementation on ViewDataDictionary, here:

public object this[string key]
{
    get
    {
        object value;
        _innerDictionary.TryGetValue(key, out value);
        return value;
    }
    set { _innerDictionary[key] = value; }
}

https://github.com/ASP-NET-MVC/aspnetwebstack/blob/master/src/System.Web.Mvc/ViewDataDictionary.cs

In other words, it always returns a value for all keys, regardless of what's in it - it simply returns null when nothing's there. But, ViewDataDictionary has the burden of being tied to MVC's Model, so it's better to strip out just the graceful dictionary parts for use outside MVC Views.

It's too long to really post all the guts here - most of it just implementing IDictionary - but here's a dynamic object (class DDict) that doesn't throw for null checks on properties that haven't been declared, on Github:

https://github.com/b9chris/GracefulDynamicDictionary

If you just want to add it to your project via NuGet, its name is GracefulDynamicDictionary.


Why did you vote down on DynamicDictionary as it does not use reflection then ?
then you can vote it up as this is the same solution :)
It most certainly is not the same solution.
THIS, THIS is exactly why I am here, I could not figure out why some code (viewbag) was NOT breaking. THANK you.
Just used your package in combination with the Newtonsoft JSON deserialiser and it worked like a charm! dynamic stuff = JsonConvert.DeserializeObject(jsonStuff, new ExpandoObjectConverter());
M
Maxime

I wanted to create an extension method so I could do something like:

dynamic myDynamicObject;
myDynamicObject.propertyName = "value";

if (myDynamicObject.HasProperty("propertyName"))
{
    //...
}

... but you can't create extensions on ExpandoObject according to the C# 5 documentation (more info here).

So I ended up creating a class helper:

public static class ExpandoObjectHelper
{
    public static bool HasProperty(ExpandoObject obj, string propertyName)
    {
        return obj != null && ((IDictionary<String, object>)obj).ContainsKey(propertyName);
    }
}

To use it:

// If the 'MyProperty' property exists...
if (ExpandoObjectHelper.HasProperty(obj, "MyProperty"))
{
    ...
}

up vote for useful comment and link on extensions for ExpandoObject.
obj could be null, hence: return obj != null && ((IDictionary)obj).ContainsKey(propertyName);
O
Oleksandr G

UPDATED: You can use delegates and try to get a value from the dynamic object property if it exists. If there is no property, simply catch the exception and return false.

Take a look, it works fine for me:

class Program
{
    static void Main(string[] args)
    {
        dynamic userDynamic = new JsonUser();

        Console.WriteLine(IsPropertyExist(() => userDynamic.first_name));
        Console.WriteLine(IsPropertyExist(() => userDynamic.address));
        Console.WriteLine(IsPropertyExist(() => userDynamic.last_name));
    }

    class JsonUser
    {
        public string first_name { get; set; }
        public string address
        {
            get
            {
                throw new InvalidOperationException("Cannot read property value");
            }
        }
    }

    static bool IsPropertyExist(GetValueDelegate getValueMethod)
    {
        try
        {
            //we're not interesting in the return value. What we need to know is whether an exception occurred or not
            getValueMethod();
            return true;
        }
        catch (RuntimeBinderException)
        {
            // RuntimeBinderException occurred during accessing the property
            // and it means there is no such property         
            return false;
        }
        catch
        {
            //property exists, but an exception occurred during getting of a value
            return true;
        }
    }

    delegate string GetValueDelegate();
}

The output of the code is the following:

True
True
False

@marklam it's bad to catch all the exception when you don't know what causes the exception. In our cases it's ok since we expect possibly absence of a field.
if you know what causes the exception you must also know its type, so catch (WhateverException) Otherwise your code will silently carry on even if you got an unexpected exception - like OutOfMemoryException for example.
You can pass in any getter to IsPropertyExist. In this example, you know one can throw an InvalidOperationException. In practice, you have no idea what exception may be thrown. +1 to counteract the cargo cult.
This solution is unacceptable if performance is important, for example if used in a loop with 500+ iterations it adds up and can cause many seconds of delay. Every time an exception is caught the stack must be copied to the exception object
Re: Performance: The debugger being attached and Console.WriteLine are the slow bits. 10,000 iterations here takes less than 200ms (with 2 exceptions per iteration). The same test with no exceptions takes a handful of milliseconds. That means if you expect your usage of this code to only rarely be missing a property, or if you're calling it a limited number of times, or can cache the results, then please realize that everything has its place and none of the overly-regurgitated warnings here matter.
C
Community

I answered a very similar question recently: How do I reflect over the members of dynamic object?

Shortly, ExpandoObject is not the only dynamic object you might get. Reflection would work for static types (types that do not implement IDynamicMetaObjectProvider). For types that do implement this interface, reflection is basically useless. For ExpandoObject, you can simply check whether the property is defined as a key in the underlying dictionary. For other implementations, it might be challenging and sometimes the only way is to work with exceptions. For details, follow the link above.


V
Vokinneberg

Why you do not want to use Reflection to get set of type properyes? Like this

 dynamic v = new Foo();
 Type t = v.GetType();
 System.Reflection.PropertyInfo[] pInfo =  t.GetProperties();
 if (Array.Find<System.Reflection.PropertyInfo>(pInfo, p => { return p.Name == "PropName"; }).    GetValue(v,  null) != null))
 {
     //PropName initialized
 } 

I'm not sure if that will return the dynamicly added properties, my guess is that it returns the methods of the Dynamic object.
r
realdanielbyrne

This extension method checks for the existence of a property and then returns the value or null. This is useful if you do not want your applications to throw unnecessary exceptions, at least ones you can help.

    public static object Value(this ExpandoObject expando, string name)
    {
        var expandoDic = (IDictionary<string, object>)expando;
        return expandoDic.ContainsKey(name) ? expandoDic[name] : null;
    }

If can be used as such :

  // lookup is type 'ExpandoObject'
  object value = lookup.Value("MyProperty");

or if your local variable is 'dynamic' you will have to cast it to ExpandoObject first.

  // lookup is type 'dynamic'
  object value = ((ExpandoObject)lookup).Value("PropertyBeingTested");

I
Ian Kemp

Depending on your use case, if null can be considered as being the same as undefined, you can turn your ExpandoObject into a DynamicJsonObject.

    dynamic x = new System.Web.Helpers.DynamicJsonObject(new ExpandoObject());
    x.a = 1;
    x.b = 2.50;
    Console.WriteLine("a is " + (x.a ?? "undefined"));
    Console.WriteLine("b is " + (x.b ?? "undefined"));
    Console.WriteLine("c is " + (x.c ?? "undefined"));

Output:

a is 1
b is 2.5
c is undefined

R
Ramy Yousef
(authorDynamic as ExpandoObject).Any(pair => pair.Key == "YourProp");

S
Softlion

Hey guys stop using Reflection for everything it costs a lots of CPU cycles.

Here is the solution:

public class DynamicDictionary : DynamicObject
{
    Dictionary<string, object> dictionary = new Dictionary<string, object>();

    public int Count
    {
        get
        {
            return dictionary.Count;
        }
    }

    public override bool TryGetMember(GetMemberBinder binder, out object result)
    {
        string name = binder.Name;

        if (!dictionary.TryGetValue(binder.Name, out result))
            result = "undefined";

        return true;
    }

    public override bool TrySetMember(SetMemberBinder binder, object value)
    {
        dictionary[binder.Name] = value;
        return true;
    }
}

This shows how to implement a dynamic object, not how to see it a property exits on a dynamic object.
You can check if a dynamic instance has a property by doing a null check against the property in question.
"This shows how to implement a dynamic object": yes in fact it is. The solution to this question is: there is no generic solution as it depends on the implementation.
@Softlion No, the solution is that thing that we have to stop using
@Softlion What is the point of the Tryxxx methods? TryGet will never return false when it doesn't find the property, so you still have to check the result. The return is useless. In TrySet, if the key does not exist, then it will throw an exception instead of returning false. I don't understand why you would even use this as an answer, if you yourself wrote here on the comments "The solution to this question is: there is no generic solution as it depends on the implementation", that's also not true. Look at Dykam's answer for the real solution.
V
Venkat

Try this one

public bool PropertyExist(object obj, string propertyName)
{
 return obj.GetType().GetProperty(propertyName) != null;
}

This would check the existence of a property of the object hidden under the dynamic name, which is an implementation detail. Have you verified your solution in real code before posting ? It should not work at all.
I have used this piece of code in real time scenario. It works good.
Gives me null all the time, even if the property exists.
It would work with basic objects, but not with ExpandoObjects. Dynamics, not sure.
To confirm, this does not work with dynamic objects either (always returns null).