问题
C#6 Update
In C#6 ?. is now a language feature:
// C#1-5
propertyValue1 = myObject != null ? myObject.StringProperty : null;
// C#6
propertyValue1 = myObject?.StringProperty;
The question below still applies to older versions, but if developing a new application using the new ?. operator is far better practice.
Original Question:
I regularly want to access properties on possibly null objects:
string propertyValue1 = null;
if( myObject1 != null )
propertyValue1 = myObject1.StringProperty;
int propertyValue2 = 0;
if( myObject2 != null )
propertyValue2 = myObject2.IntProperty;
And so on...
I use this so often that I have a snippet for it.
You can shorten this to some extent with an inline if:
propertyValue1 = myObject != null ? myObject.StringProperty : null;
However this is a little clunky, especially if setting lots of properties or if more than one level can be null, for instance:
propertyValue1 = myObject != null ?
(myObject.ObjectProp != null ? myObject.ObjectProp.StringProperty) : null : null;
What I really want is ?? style syntax, which works great for directly null types:
int? i = SomeFunctionWhichMightReturnNull();
propertyValue2 = i ?? 0;
So I came up with the following:
public static TResult IfNotNull<T, TResult>( this T input, Func<T, TResult> action, TResult valueIfNull )
where T : class
{
if ( input != null ) return action( input );
else return valueIfNull;
}
//lets us have a null default if the type is nullable
public static TResult IfNotNull<T, TResult>( this T input, Func<T, TResult> action )
where T : class
where TResult : class
{ return input.IfNotNull( action, null ); }
This lets me us this syntax:
propertyValue1 = myObject1.IfNotNull( x => x.StringProperty );
propertyValue2 = myObject2.IfNotNull( x => x.IntProperty, 0);
//or one with multiple levels
propertyValue1 = myObject.IfNotNull(
o => o.ObjectProp.IfNotNull( p => p.StringProperty ) );
This simplifies these calls, but I\'m not sure about checking this sort of extension method in - it does make the code a little easier to read, but at the cost of extending object. This would appear on everything, although I could put it in a specifically referenced namespace.
This example is a rather simple one, a slightly more complex one would be comparing two nullable object properties:
if( ( obj1 == null && obj2 == null ) ||
( obj1 != null && obj2 != null && obj1.Property == obj2.Property ) )
...
//becomes
if( obj1.NullCompare( obj2, (x,y) => x.Property == y.Property )
...
What are the pitfalls of using extensions in this way? Are other coders likely to be confused? Is this just abuse of extensions?
I guess what I really want here is a compiler/language extension:
propertyValue1 = myObject != null ? myObject.StringProperty : null;
//becomes
propertyValue1 = myObject?StringProperty;
This would make the complex case far easier:
propertyValue1 = myObject != null ?
(myObject.ObjectProp != null ? myObject.ObjectProp.StringProperty) : null
//becomes
propertyValue1 = myObject?ObjectProp?StringProperty;
This would only work for value types, but you could return nullable equivalents:
int? propertyValue2 = myObject?ObjectProp?IntProperty;
//or
int propertyValue3 = myObject?ObjectProp?IntProperty ?? 0;
回答1:
We independently came up with the exact same extension method name and implementation: Null-propagating extension method. So we don't think it's confusing or an abuse of extension methods.
I would write your "multiple levels" example with chaining as follows:
propertyValue1 = myObject.IfNotNull(o => o.ObjectProp).IfNotNull(p => p.StringProperty);
There's a now-closed bug on Microsoft Connect that proposed "?." as a new C# operator that would perform this null propagation. Mads Torgersen (from the C# language team) briefly explained why they won't implement it.
回答2:
Here's another solution, for chained members, including extension methods:
public static U PropagateNulls<T,U> ( this T obj
,Expression<Func<T,U>> expr)
{ if (obj==null) return default(U);
//uses a stack to reverse Member1(Member2(obj)) to obj.Member1.Member2
var members = new Stack<MemberInfo>();
bool searchingForMembers = true;
Expression currentExpression = expr.Body;
while (searchingForMembers) switch (currentExpression.NodeType)
{ case ExpressionType.Parameter: searchingForMembers = false; break;
case ExpressionType.MemberAccess:
{ var ma= (MemberExpression) currentExpression;
members.Push(ma.Member);
currentExpression = ma.Expression;
} break;
case ExpressionType.Call:
{ var mc = (MethodCallExpression) currentExpression;
members.Push(mc.Method);
//only supports 1-arg static methods and 0-arg instance methods
if ( (mc.Method.IsStatic && mc.Arguments.Count == 1)
|| (mc.Arguments.Count == 0))
{ currentExpression = mc.Method.IsStatic ? mc.Arguments[0]
: mc.Object;
break;
}
throw new NotSupportedException(mc.Method+" is not supported");
}
default: throw new NotSupportedException
(currentExpression.GetType()+" not supported");
}
object currValue = obj;
while(members.Count > 0)
{ var m = members.Pop();
switch(m.MemberType)
{ case MemberTypes.Field:
currValue = ((FieldInfo) m).GetValue(currValue);
break;
case MemberTypes.Method:
var method = (MethodBase) m;
currValue = method.IsStatic
? method.Invoke(null,new[]{currValue})
: method.Invoke(currValue,null);
break;
case MemberTypes.Property:
var method = ((PropertyInfo) m).GetGetMethod(true);
currValue = method.Invoke(currValue,null);
break;
}
if (currValue==null) return default(U);
}
return (U) currValue;
}
Then you can do this where any can be null, or none:
foo.PropagateNulls(x => x.ExtensionMethod().Property.Field.Method());
回答3:
If you find yourself having to check very often if a reference to an object is null, may be you should be using the Null Object Pattern. In this pattern, instead of using null to deal with the case where you don't have an object, you implement a new class with the same interface but with methods and properties that return adequate default values.
回答4:
How is
propertyValue1 = myObject.IfNotNull(o => o.ObjectProp.IfNotNull( p => p.StringProperty ) );
easier to read and write than
if(myObject != null && myObject.ObjectProp != null)
propertyValue1 = myObject.ObjectProp.StringProperty;
Jafar Husain posted a sample of using Expression Trees to check for null in a chain, Runtime macros in C# 3.
This obviously has performance implications though. Now if only we had a way to do this at compile time.
回答5:
I just have to say that I love this hack!
I hadn't realized that extension methods don't imply a null check, but it totally makes sense. As James pointed out, The extension method call itself is not any more expensive than a normal method, however if you are doing a ton of this, then it does make sense to follow the Null Object Pattern, that ljorquera suggested. Or to use a null object and ?? together.
class Class1
{
public static readonly Class1 Empty = new Class1();
.
.
x = (obj1 ?? Class1.Empty).X;
回答6:
it does make the code a little easier to read, but at the cost of extending object. This would appear on everything,
Note that you are not actually extending anything (except theoretically).
propertyValue2 = myObject2.IfNotNull( x => x.IntProperty, 0);
will generate IL code exactly as if it were written:
ExtentionClass::IfNotNull(myObject2, x => x.IntProperty, 0);
There is no "overhead" added to the objects to support this.
回答7:
To reader not in the know it looks like you're calling a method on a null reference. If you want this, I'd suggest putting it in a utility class rather than using an extension method:
propertyValue1 = Util.IfNotNull(myObject1, x => x.StringProperty );
propertyValue2 = Util.IfNotNull(myObject2, x => x.IntProperty, 0);
The "Util." grates, but is IMO the lesser syntactic evil.
Also, if you developing this as part of a team, then gently ask what others think and do. Consistency across a codebase for frequently used patterns is important.
回答8:
While extension methods generally cause misunderstandings when called from null instances, I think the intent is pretty straightforward in this case.
string x = null;
int len = x.IfNotNull(y => y.Length, 0);
I would want to be sure this static method works on Value Types that can be null, such as int?
Edit: compiler says that neither of these are valid:
public void Test()
{
int? x = null;
int a = x.IfNotNull(z => z.Value + 1, 3);
int b = x.IfNotNull(z => z.Value + 1);
}
Other than that, go for it.
回答9:
Not an answer to the exact question asked, but there is Null-Conditional Operator in C# 6.0. I can argue it will be a poor choice to use the option in OP since C# 6.0 :)
So your expression is simpler,
string propertyValue = myObject?.StringProperty;
In case myObject is null it returns null. In case the property is a value type you have to use equivalent nullable type, like,
int? propertyValue = myObject?.IntProperty;
Or otherwise you can coalesce with null coalescing operator to give a default value in case of null. For eg,
int propertyValue = myObject?.IntProperty ?? 0;
?. is not the only syntax available. For indexed properties you can use ?[..]. For eg,
string propertyValue = myObject?[index]; //returns null in case myObject is null
One surprising behaviour of the ?. operator is that it can intelligently bypass subsequent .Member calls if object happens to be null. One such example is given in the link:
var result = value?.Substring(0, Math.Min(value.Length, length)).PadRight(length);
In this case result is null if value is null and value.Length expression wouldn't result in NullReferenceException.
回答10:
Personally, even after all your explanation, I can't remember how the heck this works:
if( obj1.NullCompare( obj2, (x,y) => x.Property == y.Property )
This could be because I have no C# experience; however, I could read and understand everything else in your code. I prefer to keep code language agnostic (esp. for trivial things) so that tomorrow, another developer could change it to a whole new language without too much information about the existing language.
回答11:
Here is another solution using myObject.NullSafe(x=>x.SomeProperty.NullSafe(x=>x.SomeMethod)), explained at http://www.epitka.blogspot.com/
来源：https://stackoverflow.com/questions/123088/possible-pitfalls-of-using-this-extension-method-based-shorthand