Exception when using lambda expressions with the entire list of items #212 (#215)

Author: b0bi79

ClosedXML.Report/FormulaEvaluator.cs

@@ -1,10 +1,12 @@
 using ClosedXML.Report.Utils;
 using DocumentFormat.OpenXml.Bibliography;
 using System;
+using System.Collections;
 using System.Collections.Generic;
 using System.Globalization;
 using System.Linq;
 using System.Linq.Expressions;
+using System.Reflection;
 using System.Text.RegularExpressions;
 
 namespace ClosedXML.Report
@@ -46,7 +48,27 @@ public bool TryEvaluate(string formula, out object result, params Parameter[] pa
 
         public void AddVariable(string name, object value)
         {
-            _variables[name] = value;
+            if (value != null && !value.GetType().IsGenericType && value is IEnumerable enumerable)
+            {
+                var itemType = enumerable.GetItemType();
+                var newEnumerable = EnumerableCastTo(enumerable, itemType);
+                _variables[name] = newEnumerable;
+            }
+            else
+                _variables[name] = value;
+        }
+
+        private IEnumerable EnumerableCastTo(IEnumerable enumerable, Type itemType)
+        {
+            ParameterExpression source = Expression.Parameter(typeof(IEnumerable));
+
+            MethodInfo method = typeof(Enumerable).GetMethod(nameof(Enumerable.Cast), BindingFlags.Public | BindingFlags.Static);
+            MethodInfo castGenericMethod = method.MakeGenericMethod(itemType);
+            var castExpr = Expression.Call(null, castGenericMethod, source);
+
+            var lambda = Expression.Lambda<Func<IEnumerable, IEnumerable>>(castExpr, source);
+
+            return lambda.Compile().Invoke(enumerable);
         }
 
         private string ObjToString(object val)

ClosedXML.Report/Utils/TypeExtensions.cs

@@ -50,10 +50,18 @@ public static Type GetItemType(this IEnumerable sourceList)
             if (result != typeof(object))
                 return result;
 
-            var item = sourceList.Cast<object>().First();
+            var item = sourceList.Cast<object>().FirstOrDefault(x => x != null);
             if (item == null)
                 return typeof(object);
-            return item.GetType();
+            var itemType = item.GetType();
+            return !sourceList.IsAllItemsAssignableFrom(itemType)
+                ? typeof(object)
+                : itemType;
+        }
+
+        public static bool IsAllItemsAssignableFrom(this IEnumerable list, Type type)
+        {
+            return list.Cast<object>().All(item => item.GetType().IsAssignableFrom(type));
         }
 
         internal static bool IsNumeric(this Type type)

tests/ClosedXML.Report.Tests/FormulaEvaluatorTests.cs

@@ -153,6 +153,21 @@ public void EvalMixedArray()
             }
         }
 
+        [Fact]
+        public void UsingLambdaExpressionsIssue212()
+        {
+            var customers = new object[10000];
+            for (int i = 0; i < customers.Length; i++)
+            {
+                customers[i] = new Customer { Id = i, Name = "Customer"+i };
+            }
+
+            var eval = new FormulaEvaluator();
+            eval.AddVariable("items", customers);
+            eval.Evaluate("{{items.Count(c => c.Id == 9999)}}").Should().Be(1);
+            eval.Evaluate("{{items.Select(i => i.Name).Skip(1000).First()}}").Should().Be("Customer1000");
+        }
+
         class Customer
         {
             public int Id { get; set; }