The type string must be a non-nullable type in order to use it as parameter T in the generic type or method System Nullable T

Question

Why do I get Error  The type  string  must be a non-nullable value type in order to use it as parameter  T  in the generic type or method  System Nullable     using System  using System Collections  using System Collections Generic  using System Data  using System Diagnostics  using Universe   namespace Universe       public class clsdictionary             private string  m Word             private string  m Meaning              string  Word              get   return m Word              set   m Word   value                   string  Meaning              get   return m Meaning              set   m Meaning   value

User · Answer

string is a reference type  a class  You can only use Nullable lt T gt  or the T  C  syntactic sugar with non-nullable value types such as int and Guid   In particular  as string is a reference type  an expression of type string can already be null   string lookMaNoText   null

User · Answer

System String  with capital S  is already nullable  you do not need to declare it as such    string  myStr  is wrong

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Please note that in upcoming version of C  which is 8  the answers are not true    All the reference types are non-nullable by default and you can actually do the following    public string  MyNullableString   this MyNullableString   null    Valid   However    public string MyNonNullableString   this MyNonNullableString   null    Not Valid and you ll receive compiler warning     The important thing here is to show the intent of your code   If the  intent  is that the reference type can be null  then mark it so otherwise assigning null value to non-nullable would result in compiler warning    More info

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Use string instead of string  in all places in your code   The Nullable lt T gt  type requires that T is a non-nullable value type  for example int or DateTime  Reference types like string can already be null  There would be no point in allowing things like Nullable lt string gt  so it is disallowed   Also if you are using C  3 0 or later you can simplify your code by using auto-implemented properties   public class WordAndMeaning       public string Word   get  set        public string Meaning   get  set

User · Answer

For a very specific reason Type Nullable lt int gt  put your cursor on Nullable and hit F12 - The Metadata provides the reason  Note the struct constraint     public struct Nullable lt T gt  where T   struct           http   msdn microsoft com en-us library d5x73970 aspx

[c#] The type 'string' must be a non-nullable type in order to use it as parameter T in the generic type or method 'System.Nullable<T>'

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