Fix Average to support nullable types (#704)

Author: StefH

src/System.Linq.Dynamic.Core/Parser/TypeHelper.cs

@@ -417,6 +417,12 @@ public static string GetTypeName(Type? type)
             return name;
         }
 
+        public static Type GetNullableType(Type type)
+        {
+            type = Nullable.GetUnderlyingType(type) ?? type;
+            return type.GetTypeInfo().IsValueType ? typeof(Nullable<>).MakeGenericType(type) : type;
+        }
+
         public static Type GetNonNullableType(Type type)
         {
             Check.NotNull(type, nameof(type));

src/System.Linq.Dynamic.Core/Util/QueryableMethodFinder.cs

@@ -1,4 +1,6 @@
-using System.Linq.Expressions;
+using System.Collections.Generic;
+using System.Linq.Dynamic.Core.Parser;
+using System.Linq.Expressions;
 using System.Reflection;
 
 namespace System.Linq.Dynamic.Core.Util;
@@ -13,8 +15,16 @@ public static MethodInfo GetGenericMethod(string name)
     public static MethodInfo GetMethod(string name, Type argumentType, Type returnType, int parameterCount = 0, Func<MethodInfo, bool>? predicate = null) =>
         GetMethod(name, returnType, parameterCount, mi => mi.ToString().Contains(argumentType.ToString()) && ((predicate == null) || predicate(mi)));
 
-    public static MethodInfo GetMethod(string name, Type returnType, int parameterCount = 0, Func<MethodInfo, bool>? predicate = null) =>
-        GetMethod(name, parameterCount, mi => (mi.ReturnType == returnType) && ((predicate == null) || predicate(mi)));
+    public static MethodInfo GetMethod(string name, Type returnType, int parameterCount = 0, Func<MethodInfo, bool>? predicate = null)
+    {
+        var returnTypes = new List<Type> { returnType };
+        if (!TypeHelper.IsNullableType(returnType))
+        {
+            returnTypes.Add(TypeHelper.GetNullableType(returnType));
+        }
+
+        return GetMethod(name, parameterCount, mi => returnTypes.Contains(mi.ReturnType) && (predicate == null || predicate(mi)));
+    }
 
     public static MethodInfo GetMethodWithExpressionParameter(string name) =>
         GetMethod(name, 1, mi =>
@@ -35,7 +45,7 @@ public static MethodInfo GetMethod(string name, int parameterCount = 0, Func<Met
     {
         try
         {
-            return typeof(Queryable).GetTypeInfo().GetDeclaredMethods(name).Single(mi =>
+            return typeof(Queryable).GetTypeInfo().GetDeclaredMethods(name).First(mi =>
                 mi.GetParameters().Length == parameterCount + 1 && (predicate == null || predicate(mi)));
         }
         catch (Exception ex)

test/System.Linq.Dynamic.Core.Tests/QueryableTests.Average.cs

@@ -1,36 +1,57 @@
-using System.Linq.Dynamic.Core.Tests.Helpers.Models;
+using System.Collections.Generic;
+using System.Linq.Dynamic.Core.Tests.Helpers.Models;
 using Xunit;
 
-namespace System.Linq.Dynamic.Core.Tests
+namespace System.Linq.Dynamic.Core.Tests;
+
+public partial class QueryableTests
 {
-    public partial class QueryableTests
+    [Fact]
+    public void Average()
+    {
+        // Arrange
+        var incomes = User.GenerateSampleModels(100).Select(u => u.Income).ToArray();
+
+        // Act
+        var expected = incomes.Average();
+        var actual = incomes.AsQueryable().Average();
+
+        // Assert
+        Assert.Equal(expected, actual);
+    }
+
+    [Fact]
+    public void Average_Nullable()
+    {
+        // Arrange
+        var list = new List<Entity> { new Entity { Value = 1 }, new Entity { Value = 2 }, new Entity { Value = null } };
+        var queryable = list.AsQueryable();
+
+
+        // Act
+        var expected = queryable.Average(p => p.Value);
+        var actual = queryable.Average("Value");
+
+        // Assert
+        Assert.Equal(expected, actual);
+    }
+
+    [Fact]
+    public void Average_Selector()
+    {
+        // Arrange
+        var users = User.GenerateSampleModels(100);
+
+        // Act
+        var expected = users.Average(u => u.Income);
+        var result = users.AsQueryable().Average("Income");
+
+        // Assert
+        Assert.Equal(expected, result);
+    }
+
+    public class Entity
     {
-        [Fact]
-        public void Average()
-        {
-            // Arrange
-            var incomes = User.GenerateSampleModels(100).Select(u => u.Income);
-
-            // Act
-            var expected = incomes.Average();
-            var actual = incomes.AsQueryable().Average();
-
-            // Assert
-            Assert.Equal(expected, actual);
-        }
-
-        [Fact]
-        public void Average_Selector()
-        {
-            // Arrange
-            var users = User.GenerateSampleModels(100);
-
-            // Act
-            var expected = users.Average(u => u.Income);
-            var result = users.AsQueryable().Average("Income");
-
-            // Assert
-            Assert.Equal(expected, result);
-        }
+        public int? Value { get; set; }
     }
-}
+}