🚸Describe why Parse fails due to case-insensitive ambiguity

UnitsNet.Serialization.JsonNet/AbbreviatedUnitsConverter.cs

@@ -190,7 +190,7 @@ protected string GetQuantityType(IQuantity quantity)
         }
 
         /// <summary>
-        ///     Attempt to find an a unique (non-ambiguous) unit matching the provided abbreviation.
+        ///     Attempt to find a unique (non-ambiguous) unit matching the provided abbreviation.
         ///     <remarks>
         ///         An exhaustive search using all quantities is very likely to fail with an
         ///         <exception cref="AmbiguousUnitParseException" />, so make sure you're using the minimum set of supported quantities.

UnitsNet.Tests/QuantityParserTests.cs

@@ -1,4 +1,4 @@
-// Licensed under MIT No Attribution, see LICENSE file at the root.
+// Licensed under MIT No Attribution, see LICENSE file at the root.
 // Copyright 2013 Andreas Gullberg Larsen ([email protected]). Maintained at https://github.com/angularsen/UnitsNet.
 
 using UnitsNet.Tests.CustomQuantities;
@@ -40,7 +40,7 @@ public void Parse_WithOneCaseInsensitiveMatchAndOneExactMatch_ParsesWithTheExact
         }
 
         [Fact]
-        public void Parse_WithMultipleCaseInsensitiveMatchesButNoExactMatches_ThrowsUnitNotFoundException()
+        public void Parse_WithMultipleCaseInsensitiveMatchesButNoExactMatches_ThrowsAmbiguousUnitParseException()
         {
             var unitAbbreviationsCache = new UnitAbbreviationsCache();
             unitAbbreviationsCache.MapUnitToAbbreviation(HowMuchUnit.Some, "foo");
@@ -52,7 +52,8 @@ void Act()
                 quantityParser.Parse<HowMuch, HowMuchUnit>("1 Foo", null, (value, unit) => new HowMuch((double) value, unit));
             }
 
-            Assert.Throws<UnitNotFoundException>(Act);
+            var ex = Assert.Throws<AmbiguousUnitParseException>(Act);
+            Assert.Equal("Cannot parse \"Foo\" since it matched multiple units [Some, ATon] with case-insensitive comparison, but zero units with case-sensitive comparison. To resolve the ambiguity, pass a unit abbreviation with the correct casing.", ex.Message);
         }
 
         [Fact]

UnitsNet.Tests/UnitParserTests.cs

@@ -136,5 +136,13 @@ public void Parse_MappedCustomUnit()
 
             Assert.Equal(HowMuchUnit.Some, parsedUnit);
         }
+
+        [Fact]
+        public void Parse_LengthUnit_MM_ThrowsExceptionDescribingTheAmbiguity()
+        {
+            var ex = Assert.Throws<AmbiguousUnitParseException>(() => UnitsNetSetup.Default.UnitParser.Parse<LengthUnit>("MM"));
+            Assert.Contains("Cannot parse \"MM\" since it matched multiple units [Millimeter, Megameter] with case-insensitive comparison, but zero units with case-sensitive comparison. To resolve the ambiguity, pass a unit abbreviation with the correct casing.", ex.Message);
+        }
+
     }
 }

UnitsNet/CustomCode/UnitParser.cs

@@ -72,14 +72,19 @@ public Enum Parse(string? unitAbbreviation, Type unitType, IFormatProvider? form
             var stringUnitPairs = _unitAbbreviationsCache.GetStringUnitPairs(enumValues, formatProvider);
             var matches = stringUnitPairs.Where(pair => pair.Item1.Equals(unitAbbreviation, StringComparison.OrdinalIgnoreCase)).ToArray();
 
+            // No match? Retry after normalizing the unit abbreviation.
             if(matches.Length == 0)
             {
                 unitAbbreviation = NormalizeUnitString(unitAbbreviation);
                 matches = stringUnitPairs.Where(pair => pair.Item1.Equals(unitAbbreviation, StringComparison.OrdinalIgnoreCase)).ToArray();
             }
 
-            // Narrow the search if too many hits, for example Megabar "Mbar" and Millibar "mbar" need to be distinguished
-            if(matches.Length > 1)
+            var caseInsensitiveMatches = matches;
+
+            // More than one case-insensitive match? Retry with case-sensitive match.
+            // For example, Megabar "Mbar" and Millibar "mbar" need to be distinguished.
+            bool hasMultipleCaseInsensitiveMatches = matches.Length > 1;
+            if (hasMultipleCaseInsensitiveMatches)
                 matches = stringUnitPairs.Where(pair => pair.Item1.Equals(unitAbbreviation)).ToArray();
 
             switch(matches.Length)
@@ -88,11 +93,18 @@ public Enum Parse(string? unitAbbreviation, Type unitType, IFormatProvider? form
                     return (Enum)Enum.ToObject(unitType, matches[0].Unit);
                 case 0:
                     // Retry with fallback culture, if different.
-                    if(!Equals(formatProvider, UnitAbbreviationsCache.FallbackCulture))
+                    if (formatProvider != null && !Equals(formatProvider, UnitAbbreviationsCache.FallbackCulture))
                     {
                         return Parse(unitAbbreviation, unitType, UnitAbbreviationsCache.FallbackCulture);
                     }
 
+                    if (hasMultipleCaseInsensitiveMatches)
+                    {
+                        string ciUnitsCsv = string.Join(", ", caseInsensitiveMatches.Select(x => Enum.GetName(unitType, x.Unit)));
+                        throw new AmbiguousUnitParseException(
+                            $"Cannot parse \"{unitAbbreviation}\" since it matched multiple units [{ciUnitsCsv}] with case-insensitive comparison, but zero units with case-sensitive comparison. To resolve the ambiguity, pass a unit abbreviation with the correct casing.");
+                    }
+
                     throw new UnitNotFoundException($"Unit not found with abbreviation [{unitAbbreviation}] for unit type [{unitType}].");
                 default:
                     string unitsCsv = string.Join(", ", matches.Select(x => Enum.GetName(unitType, x.Unit)).ToArray());