Optimized AddRows, no more dictionary lookup per every item added

Author: alex-jitbit

CsvExport/CsvExport.cs

@@ -130,13 +130,18 @@ public object this[string field]
 					_fields.Add(field, num);
 				}
 
-				while (num >= _currentRow.Count) //fill the current row with nulls until we have the right size
-					_currentRow.Add(null);
-
-				_currentRow[num] = value; //set the raw value at position
+				SetCurrentRowValue(num, value);
 			}
 		}
 
+		private void SetCurrentRowValue(int num, object value)
+		{
+			while (num >= _currentRow.Count) //fill the current row with nulls until we have the right size
+				_currentRow.Add(null);
+
+			_currentRow[num] = value; //set the raw value at position
+		}
+
 		/// <summary>
 		/// Call this before setting any fields on a row
 		/// </summary>
@@ -155,12 +160,28 @@ public void AddRows<T>(IEnumerable<T> list)
 			if (!e.MoveNext()) return; //empty - skip reflection cache warm-up
 
 			var accessors = ReflectionCache<T>.Accessors;
+
+			// Resolve property names to CSV column indexes once for this batch.
+			// The row loop below can then write by index instead of doing a dictionary lookup per cell.
+			var fieldIndexes = new int[accessors.Length];
+			for (int i = 0; i < accessors.Length; i++)
+			{
+				var field = accessors[i].Name;
+				if (!_fields.TryGetValue(field, out int num))
+				{
+					num = _fields.Count;
+					_fields.Add(field, num);
+				}
+
+				fieldIndexes[i] = num;
+			}
+
 			do
 			{
 				AddRow();
-				foreach (var a in accessors)
+				for (int i = 0; i < accessors.Length; i++)
 				{
-					this[a.Name] = a.Getter(e.Current);
+					SetCurrentRowValue(fieldIndexes[i], accessors[i].Getter(e.Current));
 				}
 			} while (e.MoveNext());
 		}

UnitTests/UnitTests.cs

@@ -82,6 +82,22 @@ public void TestGeneric()
 			Assert.IsTrue(csv.Trim() == "sep=,\r\nId,Name\r\n123,Ffff\r\n321,ddd", csv);
 		}
 
+		[TestMethod]
+		public void TestGenericAfterExistingColumn()
+		{
+			List<MyClass> list = new() { new MyClass { Id = 123, Name = "Ffff" } };
+
+			var myExport = new CsvExport();
+			myExport.AddRow();
+			myExport["Prefix"] = "manual";
+
+			myExport.AddRows(list);
+
+			string csv = myExport.Export();
+
+			Assert.IsTrue(csv.Trim() == "sep=,\r\nPrefix,Id,Name\r\nmanual\r\n,123,Ffff", csv);
+		}
+
 		[TestMethod]
 		public void TestEmpty()
 		{
@@ -389,4 +405,4 @@ public class MyClass
 		public int Id { get; set; }
 		public string Name { get; set; } = string.Empty;
 	}
-}
+}