Null generic

If the type of T is nonnullable then the null state is "not null". Otherwise the null state depends on the conversion from the type of E to type T :. The null state of E1? E2 : E3 is based on the null state of E2 and E3 :. E , checked E and unchecked E all have the same null state as E.

Nested functions lambdas and local functions are treated like methods, except in regards to their captured variables. The initial state of a captured variable inside a lambda or local function is the intersection of the nullable state of the variable at all the "uses" of that nested function or lambda.

A use of a local function is either a call to that function, or where it is converted to a delegate. A use of a lambda is the point at which it is defined in source. For instance:. Generic type inference is enhanced to help decide whether inferred reference types should be nullable or not. This is a best effort. It may yield warnings regarding nullability constraints, and may lead to nullable warnings when the inferred types of the selected overload are applied to the arguments.

Nullable reference types flow into the bounds from the initial expressions, as described below. In addition, two new kinds of bounds, namely null and default are introduced.

Their purpose is to carry through occurrences of null or default in the input expressions, which may cause an inferred type to be nullable, even when it otherwise wouldn't. If an argument Ei has a reference type, the type U used for inference depends on the null state of Ei as well as its declared type:. In inferences from the type U to the type V , if V is a nullable reference type V0?

The essence is that nullability that pertains directly to one of the unfixed type variables is preserved into its bounds. For the inferences that recurse further into the source and target types, on the other hand, nullability is ignored.

It may or may not match, but if it doesn't, a warning will be issued later if the overload is chosen and applied. The spec currently does not do a good job of describing what happens when multiple bounds are identity convertible to each other, but are different.

This may happen between object and dynamic , between tuple types that differ only in element names, between types constructed thereof and now also between C and C? In addition we need to propagate "nullness" from the input expressions to the result type. Merging is described between two candidate types.

It is transitive and commutative, so the candidates can be merged in any order with the same ultimate result. It is undefined if the two candidate types are not identity convertible to each other. Skip to main content. This browser is no longer supported.

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Microsoft makes no warranties, express or implied, with respect to the information provided here. The following code example defines three rows of a table in the Microsoft Pubs sample database.

The table contains two columns that are not nullable and two columns that are nullable. A type is said to be nullable if it can be assigned a value or can be assigned null , which means the type has no value whatsoever. By default, all reference types, such as String , are nullable, but all value types, such as Int32 , are not.

In C and Visual Basic, you mark a value type as nullable by using the? For example, int? The Nullable class supports obtaining the underlying type of a nullable type, and comparison and equality operations on pairs of nullable types whose underlying value type does not support generic comparison and equality operations. If the HasValue property is false , the value of the object is undefined and an attempt to access the Value property throws an InvalidOperationException.

That is, if the HasValue property is true , the contents of the Value property is boxed. Your other option would be to to add this to the end of your declaration: where T : class where T: IList That way it will allow you to return null. If both constraints are to the same type, you mention the type once and use a comma, like where T : class, IList.

For completeness sake, it's good to know you could also do this: return default; It returns the same as return default T ;. Otherwise, potentially consider an output parameter for "match found". Here's a working example for Nullable Enum return values: public static TEnum? Since C 7.

This ensures that a caller does not accidentally supply a value type that is not an enum such as an int or a DateTime. Did you try this code? Only return default T ; will work. Shcherbo nope it work. Try it on fiddle. Okay, I didn't notice in your answer that your IThing is the class so your method knows that T is the class, but I'm sure that the author of the question means that IThing is an interface what the I prefix is saying about.

Shcherbo I have to apologize. You have right. Unfortunately question is not very verbose. Therefore for lazy people as me is not very feasible to achieve same conditions. It showed the important point how the behaviour of a generic method differs when a type argument is known to be a class or an interface and also emphasised the fact that where T : IThing is also fine with value types.

Jeson Martajaya. Am I right? The Overflow Blog. Podcast Making Agile work for data science. Stack Gives Back Featured on Meta. New post summary designs on greatest hits now, everywhere else eventually. Linked 5. See more linked questions. Related Hot Network Questions.