Fix: Throw on generic context GetTypeArgument + Add: GenericContext From methods

src/AsmResolver.DotNet/Signatures/GenericContext.cs

@@ -1,10 +1,12 @@
 using System;
+using System.ComponentModel;
+using System.Diagnostics.CodeAnalysis;
 using AsmResolver.DotNet.Signatures.Types;
 
 namespace AsmResolver.DotNet.Signatures
 {
     /// <summary>
-    /// Provides a context within a generic instantiation, including the type arguments of the enclosing type and method. 
+    /// Provides a context within a generic instantiation, including the type arguments of the enclosing type and method.
     /// </summary>
     public readonly struct GenericContext
     {
@@ -18,32 +20,37 @@ public GenericContext(IGenericArgumentsProvider? type, IGenericArgumentsProvider
             Type = type;
             Method = method;
         }
-        
+
         /// <summary>
-        /// Gets the object responsible for providing type arguments defined by the current generic type instantiation. 
+        /// Gets the object responsible for providing type arguments defined by the current generic type instantiation.
         /// </summary>
         public IGenericArgumentsProvider? Type
         {
             get;
         }
 
         /// <summary>
-        /// Gets the object responsible for providing type arguments defined by the current generic method instantiation. 
+        /// Gets the object responsible for providing type arguments defined by the current generic method instantiation.
         /// </summary>
         public IGenericArgumentsProvider? Method
         {
             get;
         }
 
         /// <summary>
-        /// Enters a new generic context with a new type providing type arguments. 
+        /// Returns true if both Type and Method providers are null
+        /// </summary>
+        public bool IsEmpty => Type is null && Method is null;
+
+        /// <summary>
+        /// Enters a new generic context with a new type providing type arguments.
         /// </summary>
         /// <param name="type">The new type providing the type arguments.</param>
         /// <returns>The new generic context.</returns>
         public GenericContext WithType(IGenericArgumentsProvider type) => new GenericContext(type, Method);
-        
+
         /// <summary>
-        /// Enters a new generic context with a new method providing type arguments. 
+        /// Enters a new generic context with a new method providing type arguments.
         /// </summary>
         /// <param name="method">The new method providing the type arguments.</param>
         /// <returns>The new generic context.</returns>
@@ -52,6 +59,11 @@ public IGenericArgumentsProvider? Method
         /// <summary>
         /// Resolves a type parameter to a type argument, based on the current generic context.
         /// </summary>
+        /// <remarks>
+        /// If a type parameter within the signature references a parameter that is not captured by the context
+        /// (i.e. the corresponding generic argument provider is set to null),
+        /// then this type parameter will not be substituted.
+        /// </remarks>
         /// <param name="parameter">The parameter to get the argument value for.</param>
         /// <returns>The argument type.</returns>
         public TypeSignature GetTypeArgument(GenericParameterSignature parameter)
@@ -62,14 +74,108 @@ public TypeSignature GetTypeArgument(GenericParameterSignature parameter)
                 GenericParameterType.Method => Method,
                 _ => throw new ArgumentOutOfRangeException()
             };
-            
+
             if (argumentProvider is null)
-                throw new ArgumentOutOfRangeException();
+                return parameter;
 
             if (parameter.Index >= 0 && parameter.Index < argumentProvider.TypeArguments.Count)
                 return argumentProvider.TypeArguments[parameter.Index];
 
             throw new ArgumentOutOfRangeException();
         }
+
+
+        /// <summary>
+        /// Gets a type generic context from <see cref="TypeSpecification"/>.
+        /// </summary>
+        /// <param name="type">Type specification to get the generic context from.</param>
+        /// <returns>Generic context.</returns>
+        public static GenericContext FromType(TypeSpecification type) => type.Signature switch
+        {
+            GenericInstanceTypeSignature typeSig => new GenericContext(typeSig, null),
+            _ => default
+        };
+
+        /// <summary>
+        /// Gets a type generic context from <see cref="GenericInstanceTypeSignature"/>.
+        /// </summary>
+        /// <param name="type">Generic type signature to get the generic context from.</param>
+        /// <returns>Generic context.</returns>
+        public static GenericContext FromType(GenericInstanceTypeSignature type) => new(type, null);
+
+        /// <summary>
+        /// Gets a type generic context from <see cref="ITypeDescriptor"/>.
+        /// </summary>
+        /// <param name="type">Type to get the generic context from.</param>
+        /// <returns>Generic context.</returns>
+        public static GenericContext FromType(ITypeDescriptor type) => type switch
+        {
+            TypeSpecification typeSpecification => FromType(typeSpecification),
+            GenericInstanceTypeSignature typeSig => FromType(typeSig),
+            _ => default
+        };
+
+        /// <summary>
+        /// Gets a method and/or type generic context from <see cref="MethodSpecification"/>.
+        /// </summary>
+        /// <param name="method">Method specification to get the generic context from.</param>
+        /// <returns>Generic context.</returns>
+        public static GenericContext FromMethod(MethodSpecification method)
+        {
+            return method.DeclaringType is TypeSpecification {Signature: GenericInstanceTypeSignature typeSig}
+                ? new GenericContext(typeSig, method.Signature)
+                : new GenericContext(null, method.Signature);
+        }
+
+        /// <summary>
+        /// Gets a method and/or type generic context from <see cref="IMethodDescriptor"/>.
+        /// </summary>
+        /// <param name="method">Method to get the generic context from.</param>
+        /// <returns>Generic context.</returns>
+        public static GenericContext FromMethod(IMethodDescriptor method) =>
+            method switch
+            {
+                MethodSpecification methodSpecification => FromMethod(methodSpecification),
+                MemberReference member => FromMember(member),
+                _ => default
+            };
+
+        /// <summary>
+        /// Gets a type generic context from <see cref="IFieldDescriptor"/>.
+        /// </summary>
+        /// <param name="field">Field to get the generic context from.</param>
+        /// <returns>Generic context.</returns>
+        public static GenericContext FromField(IFieldDescriptor field) =>
+            field switch
+            {
+                MemberReference member => FromMember(member),
+                _ => default
+            };
+
+        /// <summary>
+        /// Gets a type generic context from <see cref="MemberReference"/>.
+        /// </summary>
+        /// <param name="member">Member reference to get the generic context from.</param>
+        /// <returns>Generic context.</returns>
+        public static GenericContext FromMember(MemberReference member) =>
+            member.DeclaringType switch
+            {
+                TypeSpecification type => FromType(type),
+                _ => default
+            };
+
+        /// <summary>
+        /// Gets a type generic context from <see cref="IMemberDescriptor"/>.
+        /// </summary>
+        /// <param name="member">Member to get the generic context from.</param>
+        /// <returns>Generic context.</returns>
+        public static GenericContext FromMember(IMemberDescriptor member) =>
+            member switch
+            {
+                IMethodDescriptor method => FromMethod(method),
+                IFieldDescriptor field => FromField(field),
+                ITypeDescriptor type => FromType(type),
+                _ => default
+            };
     }
 }