#518 - improved handling of decimals in Concat operations during calculation

Author: swmal

src/EPPlus/FormulaParsing/Excel/Operators/Operator.cs

@@ -198,6 +198,18 @@ public static IOperator Exp
             }
         }
 
+        private static string CompileResultToString(CompileResult c)
+        {
+            if(c != null && c.IsNumeric)
+            {
+                if(c.ResultNumeric is double d)
+                {
+                    return d.ToString("G15");
+                }
+            }
+            return c.ResultValue.ToString();
+        }
+
         public static IOperator Concat
         {
             get
@@ -206,8 +218,8 @@ public static IOperator Concat
                     {
                         l = l ?? new CompileResult(string.Empty, DataType.String);
                         r = r ?? new CompileResult(string.Empty, DataType.String);
-                        var lStr = l.Result != null ? l.ResultValue.ToString() : string.Empty;
-                        var rStr = r.Result != null ? r.ResultValue.ToString() : string.Empty;
+                        var lStr = l.Result != null ? CompileResultToString(l) : string.Empty;
+                        var rStr = r.Result != null ? CompileResultToString(r) : string.Empty;
                         return new CompileResult(string.Concat(lStr, rStr), DataType.String);
                     });
             }

src/EPPlus/FormulaParsing/ExpressionGraph/ExpressionConverter.cs

@@ -23,7 +23,16 @@ public class ExpressionConverter : IExpressionConverter
         public StringExpression ToStringExpression(Expression expression)
         {
             var result = expression.Compile();
-            var newExp = new StringExpression(result.Result.ToString());
+            string toString;
+            if(result.DataType == DataType.Decimal)
+            {
+                toString = result.ResultNumeric.ToString("G15");
+            }
+            else
+            {
+                toString = result.Result.ToString();
+            }
+            var newExp = new StringExpression(toString);
             newExp.Operator = expression.Operator;
             return newExp;
         }

src/EPPlusTest/FormulaParsing/IntegrationTests/OperatorsTests.cs

@@ -59,5 +59,15 @@ public void DivByZeroShouldReturnError()
             var result = _ws.Calculate("10/0 + 3");
             Assert.AreEqual(DivByZero, result);
         }
+
+        [TestMethod]
+        public void ConcatShouldUseFormatG15()
+        {
+            var result = _ws.Calculate("14.000000000000002 & \"%\"");
+            Assert.AreEqual("14%", result);
+
+            result = _ws.Calculate("\"%\" & 14.000000000000002");
+            Assert.AreEqual("%14", result);
+        }
     }
 }