move max and min value checks into IsFirstForm check, add blank line above comment in test

Author: lukasvosyka

src/MongoDB.Bson/ObjectModel/Decimal128.cs

@@ -739,16 +739,18 @@ public static byte ToByte(Decimal128 d)
         /// <returns>A <see cref="decimal"/> equivalent to <paramref name="d" />.</returns>
         public static decimal ToDecimal(Decimal128 d)
         {
-            if (d == Decimal128.MaxValue)
-            {
-                return decimal.MaxValue;
-            }
-            else if (d == Decimal128.MinValue)
-            {
-                return decimal.MinValue;
-            }
             if (Flags.IsFirstForm(d._highBits))
             {
+                // fix for CSHARP-5792, make sure that Decimal128.MaxValue and MinValue are mapped back to
+                // decimal.MaxValue and MinValue.
+                if (d == Decimal128.MaxValue)
+                {
+                    return decimal.MaxValue;
+                }
+                if (d == Decimal128.MinValue)
+                {
+                    return decimal.MinValue;
+                }
                 if (Decimal128.Compare(d, __minDecimalValue) < 0 || Decimal128.Compare(d, __maxDecimalValue) > 0)
                 {
                     throw new OverflowException("Value is too large or too small to be converted to a Decimal.");

tests/MongoDB.Bson.Tests/Serialization/Options/RepresentationConverterTests.cs

@@ -123,6 +123,7 @@ public void TestConversions()
             // general decimal <-> Decimal128 checks
             Assert.Equal(123.45m, converter.ToDecimal(new Decimal128(123.45m)));
             Assert.Equal(-123.45m, converter.ToDecimal(new Decimal128(-123.45m)));
+
             // System.Decimal should be mapped to Decimal128.MaxValue and vice versa
             Assert.Equal(Decimal128.MaxValue, converter.ToDecimal128(decimal.MaxValue));
             Assert.Equal(Decimal128.MinValue, converter.ToDecimal128(decimal.MinValue));