I have the following ugly code:
if (msg == null ||
msg.Content == null ||
msg.Content.AccountMarketMessage == null ||
msg.Content.AccountMarketMessage.Account == null ||
msg.Content.AccountMarketMessage.Account.sObject == null) return;
Is there a way to chain check for null values in C#, so that I don't have to check each individual level?
One of the proposals in C# 6 would be to add a new Null Propogation operator.
This will (hopefully) allow you to write:
var obj = msg?.Content?.AccountMarketMessage?.Account?.sObject;
if (obj == null) return;
Unfortunately, there is nothing in the language at this point that handles this.
There is not currently such a thing, but it may be coming to .NET very soon. There is a well-known User Voice thread on the subject. And as noted in this article, the Visual Studio team has recently announced that:
We are seriously considering this feature for C# and VB, and will be prototyping it in coming months.
Edit: and as noted in Reed Copsey's answer above, it is now a planned addition for C# 6. There are better details on the Codeplex pages he linked.
There is no built-in support for this, but you can use an extension method for that:
public static bool IsNull<T>(this T source, string path)
{
var props = path.Split('.');
var type = source.GetType();
var currentObject = type.GetProperty(props[0]).GetValue(source);
if (currentObject == null) return true;
foreach (var prop in props.Skip(1))
{
currentObject = currentObject.GetType()
.GetProperty(prop)
.GetValue(currentObject);
if (currentObject == null) return true;
}
return false;
}
Then call it:
if ( !msg.IsNull("Content.AccountMarketMessage.Account.sObject") ) return;
You need monads and Monadic null checking. Could have a look at Monads.Net package. It can help with simplifying null tests and getting values from deep navigation properties
Something like
var sObject = person.With(p=>p.Content).With(w=>w.AccountMarketMessage ).With(p=>p.Account).With(p=>p.Object);
If you wanted a default value then
var sObject = person.With(p=>p.Content).With(w=>w.AccountMarketMessage).With(p=>p.Account).Return(p=>p.Object, "default value");
You can lazily evaluate the values using lambda expressions. This is overkill for a simple null check, but can be useful for chaining more complex expressions in a "fluent" manner.
// a type that has many descendents
var nested = new Nested();
// setup an evaluation chain
var isNull =
NullCheck.Check( () => nested )
.ThenCheck( () => nested.Child )
.ThenCheck( () => nested.Child.Child )
.ThenCheck( () => nested.Child.Child.Child )
.ThenCheck( () => nested.Child.Child.Child.Child );
// handle the results
Console.WriteLine( isNull.IsNull ? "null" : "not null" );
This is a full example (albeit draft-quality code) that can be pasted into a console app or LINQPad.
public class Nested
{
public Nested Child
{
get;
set;
}
}
public class NullCheck
{
public bool IsNull { get; private set; }
// continues the chain
public NullCheck ThenCheck( Func<object> test )
{
if( !IsNull )
{
// only evaluate if the last state was "not null"
this.IsNull = test() == null;
}
return this;
}
// starts the chain (convenience method to avoid explicit instantiation)
public static NullCheck Check( Func<object> test )
{
return new NullCheck { IsNull = test() == null };
}
}
private void Main()
{
// test 1
var nested = new Nested();
var isNull =
NullCheck.Check( () => nested )
.ThenCheck( () => nested.Child )
.ThenCheck( () => nested.Child.Child )
.ThenCheck( () => nested.Child.Child.Child )
.ThenCheck( () => nested.Child.Child.Child.Child );
Console.WriteLine( isNull.IsNull ? "null" : "not null" );
// test 2
nested = new Nested { Child = new Nested() };
isNull = NullCheck.Check( () => nested ).ThenCheck( () => nested.Child );
Console.WriteLine( isNull.IsNull ? "null" : "not null" );
// test 3
nested = new Nested { Child = new Nested() };
isNull = NullCheck.Check( () => nested ).ThenCheck( () => nested.Child ).ThenCheck( () => nested.Child.Child );
Console.WriteLine( isNull.IsNull ? "null" : "not null" );
}
Again: you probably shouldn't use this in lieu of simple null checks due to the complexity it introduces, but it's an interesting pattern.
As stated there is a plan to have c# 6.0 implement the ? operator to facilitate this process somewhat. If you cannot wait, I would suggest using a lambda expression and a simple helper function to solve this.
public E NestedProperty<T,E>(T Parent, Func<T,E> Path, E IfNullOrEmpty = default(E))
{
try
{
return Path(Parent);
}
catch
{
return IfNullOrEmpty;
}
}
This could be used int value = NestedProperty<First,int>(blank,f => f.Second.Third.id); as shown in the demo below
program
public class Program
{
public void Main()
{
First blank = new First();
First populated = new First(true);
//where a value exists
int value = NestedProperty<First,int>(blank,f => f.Second.Third.id);
Console.WriteLine(value);//0
//where no value exists
value = NestedProperty<First,int>(populated,f => f.Second.Third.id);
Console.WriteLine(value);//1
//where no value exists and a default was used
value = NestedProperty<First,int>(blank,f => f.Second.Third.id,-1);
Console.WriteLine(value);//-1
}
public E NestedProperty<T,E>(T Parent, Func<T,E> Path, E IfNullOrEmpty = default(E))
{
try
{
return Path(Parent);
}
catch
{
return IfNullOrEmpty;
}
}
}
simple demo structure
public class First
{
public Second Second { get; set; }
public int id { get; set; }
public First(){}
public First(bool init)
{
this.id = 1;
this.Second = new Second();
}
}
public class Second
{
public Third Third { get; set; }
public int id { get; set; }
public Second()
{
this.id = 1;
this.Third = new Third();
}
}
public class Third
{
public int id { get; set; }
public Third()
{
this.id = 1;
}
}
Since 3.5 (maybe earlier), You could write very simple extension method
public static TResult DefaultOrValue<T, TResult> (this T source,
Func<T, TResult> property) where T : class
{
return source == null ? default(TResult) : property(source);
}
You may name this method even shortier and then use like this
var instance = new First {SecondInstance = new Second
{ThirdInstance = new Third {Value = 5}}};
var val =
instance .DefaultOrValue(x => x.SecondInstance)
.DefaultOrValue(x => x.ThirdInstance)
.DefaultOrValue(x => x.Value);
Console.WriteLine(val);
Console.ReadLine();
so source classes are:
public class Third
{
public int Value;
}
public class First
{
public Second SecondInstance;
}
public class Second
{
public Third ThirdInstance;
}
