The type arguments for method cannot be inferred from the usage

Question

Maybe I m overworked  but this isn t compiling  CS0411   Why   interface ISignatur lt T gt        Type Type   get       interface IAccess lt S  T gt  where S   ISignatur lt T gt        S Signature   get            T Value   get  set       class Signatur   ISignatur lt bool gt        public Type Type               get   return typeof bool              class ServiceGate       public IAccess lt S  T gt  Get lt S  T gt  S sig  where S   ISignatur lt T gt                throw new NotImplementedException             static class Test       static void Main                 ServiceGate service   new ServiceGate            var access   service Get new Signatur        CS4011 error           Anyone an idea why not  Or how to solve

User · Accepted Answer

Get lt S  T gt  takes two type arguments   When you call service Get new Signatur     how does the compiler know what T is   You ll have to pass it explicitly or change something else about your type hierarchies    Passing it explicitly would look like   service Get lt Signatur  bool gt  new Signatur

User · Answer

I wanted to make a simple and understandable example  if you call a method like this  your client will not know return type  var interestPoints   Mediator Handle new InterestPointTypeRequest                               LanguageCode   request LanguageCode                  AgentId   request AgentId                  InterestPointId   request InterestPointId                    Then you should say to compiler i know the return type is List lt InterestPointTypeMap gt   var interestPoints    Mediator Handle lt List lt InterestPointTypeMap gt  gt  new InterestPointTypeRequest                               LanguageCode   request LanguageCode                  AgentId   request AgentId                  InterestPointId   request InterestPointId                  InterestPointTypeId   request InterestPointTypeId                   the compiler will no longer be mad at you for knowing the return type

User · Answer

Kirk s answer is right on  As a rule  you re not going to have any luck with type inference when your method signature has fewer types of parameters than it has generic type parameters   In your particular case  it seems you could possibly move the T type parameter to the class level and then get type inference on your Get method   class ServiceGate lt T gt        public IAccess lt S  T gt  Get lt S gt  S sig  where S   ISignatur lt T gt                throw new NotImplementedException              Then the code you posted with the CS0411 error could be rewritten as   static void Main            Notice  a bit more cumbersome to write here        ServiceGate lt SomeType gt  service   new ServiceGate lt SomeType gt                but at least you get type inference here      IAccess lt Signatur  SomeType gt  access   service Get new Signatur

User · Answer

Now my aim was to have one pair with an base type and a type definition  Requirement A   For the type definition I want to use inheritance  Requirement B   The use should be possible  without explicite knowledge over the base type  Requirement C     After I know now that the gernic constraints are not used for solving the generic return type  I experimented a little bit   Ok let s introducte Get2   class ServiceGate       public IAccess lt C  T gt  Get1 lt C  T gt  C control  where C   ISignatur lt T gt                throw new NotImplementedException               public IAccess lt ISignatur lt T gt   T gt  Get2 lt T gt  ISignatur lt T gt  control                throw new NotImplementedException             class Test       static void Main                 ServiceGate service   new ServiceGate              var bla1   service Get1 new Signatur        CS0411         var bla   service Get2 new Signatur        Works           Fine  but this solution reaches not requriement B    Next try   class ServiceGate       public IAccess lt C  T gt  Get3 lt C  T gt  C control  ISignatur lt T gt  iControl  where C   ISignatur lt T gt                throw new NotImplementedException              class Test       static void Main                 ServiceGate service   new ServiceGate              var bla1   service Get1 new Signatur        CS0411         var bla   service Get2 new Signatur        Works         var c   new Signatur            var bla3   service Get3 c  c      Works              Nice  Now the compiler can infer the generic return types  But i don t like it   Other try   class IC lt A  B gt        public IC A a  B b                Value1   a          Value2   b             public A Value1   get  set         public B Value2   get  set       class Signatur   ISignatur lt bool gt        public string Test   get  set         public IC lt Signatur  ISignatur lt bool gt  gt  Get                 return new IC lt Signatur  ISignatur lt bool gt  gt  this  this            class ServiceGate       public IAccess lt C  T gt  Get4 lt C  T gt  IC lt C  ISignatur lt T gt  gt  control  where C   ISignatur lt T gt                throw new NotImplementedException             class Test       static void Main                 ServiceGate service   new ServiceGate              var bla1   service Get1 new Signatur        CS0411         var bla   service Get2 new Signatur        Works         var c   new Signatur            var bla3   service Get3 c  c      Works           var bla4   service Get4  new Signatur    Get        Better              My final solution is to have something like ISignature lt B  C gt   where B ist the base type and C the definition

User · Answer

As I mentioned in my comment  I think the reason why this doesn t work is because the compiler can t infer types based on generic constraints   Below is an alternative implementation that will compile   I ve revised the IAccess interface to only have the T generic type parameter   interface ISignatur lt T gt        Type Type   get       interface IAccess lt T gt        ISignatur lt T gt  Signature   get        T Value   get  set       class Signatur   ISignatur lt bool gt        public Type Type               get   return typeof bool              class ServiceGate       public IAccess lt T gt  Get lt T gt  ISignatur lt T gt  sig                throw new NotImplementedException             static class Test       static void Main                 ServiceGate service   new ServiceGate            var access   service Get new Signatur

User · Answer

For those who are wondering why this works in Java but not C   consider what happens if some doof wrote this class  public class Trololol   ISignatur lt bool gt   ISignatur lt int gt       Type ISignatur lt bool gt  Type   gt  typeof bool       Type ISignatur lt int gt  Type   gt  typeof int      How is the compiler supposed to resolve var access   service Get new Trololol     Both int and bool are valid  The reason this implicit resolution works in Java likely has to do with Erasure and how Java will throw a fit if you try to implement an interface with two or more different type arguments  Such a class is simply not allowed in Java  but is just fine in C

User · Answer

I received this error because I had made a mistake in the definition of my method  I had declared the method to accept a generic type  notice the  quot T quot  after the method name   protected int InsertRecord lt T gt  CoasterModel model  IDbConnection con   However  when I called the method  I did not use the type which  in my case  was the correct usage  int count   InsertRecord databaseToMigrateFrom  con    I just removed the generic casting and it worked

[c#] The type arguments for method cannot be inferred from the usage

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