Added short and long tons.

Src/UnitsNet/Mass.cs

@@ -39,6 +39,16 @@ private Mass(double kilograms)
             Kilograms = kilograms;
         }
 
+        public double ShortTons
+        {
+            get { return Kilograms*907.18474; }
+        }
+
+        public double LongTons
+        {
+            get { return Kilograms*1016.0469088; }
+        }
+
         public double Megatonnes
         {
             get { return Kilograms*1E-9; }
@@ -141,6 +151,16 @@ public static Mass FromGravitationalForce(Force f)
             return new Mass(f.KilogramForce);
         }
 
+        public static Mass FromLongTons(double value)
+        {
+            return new Mass(value*0.000984207);
+        }
+
+        public static Mass FromShortTons(double value)
+        {
+            return new Mass(value*0.00110231);
+        }
+
         #endregion
 
         #region Arithmetic operators

Src/UnitsNet/Unit.cs

@@ -54,6 +54,10 @@ public enum Unit
         //Microgram,
         //Nanogram,
 
+        // Mass (imperial)
+        ShortTon,
+        LongTon,
+
         // Pressure
         Pascal,
         KiloPascal,

Src/UnitsNet/UnitConverter.cs

@@ -257,6 +257,14 @@ private static bool TryConvertMass(double value, Unit fromUnit, Unit toUnit, out
                     return true;
                     //case Unit.Microgram:
                     //case Unit.Nanogram:
+
+                case Unit.ShortTon:
+                    newValue = Convert(Mass.FromShortTons(value), toUnit);
+                    return true;
+                case Unit.LongTon:
+                    newValue = Convert(Mass.FromLongTons(value), toUnit);
+                    return true;
+
                 default:
                     newValue = 0;
                     return false;
@@ -325,6 +333,12 @@ private static double Convert(Mass m, Unit toUnit)
                     //             return m.Micrograms;
                     //case Unit.Nanogram:
                     //             return m.Nanograms;
+
+                case Unit.ShortTon:
+                    return m.ShortTons;
+                case Unit.LongTon:
+                    return m.LongTons;
+
                 default:
                     throw new Exception(
                         string.Format("Conversion from mass to unit [{0}] is either not valid or not yet implemented.",

Tests/MassTests.cs

@@ -4,7 +4,9 @@ namespace UnitsNet.Tests.net35
 {
     [TestFixture]
     public class MassTests
-    { 
+    {
+        private const double Delta = 1E-5;
+
         [Test]
         public void KilogramToMassUnits()
         { 
@@ -20,6 +22,9 @@ public void KilogramToMassUnits()
             Assert.AreEqual(1E4, oneKg.Decigrams);
             Assert.AreEqual(1E5, oneKg.Centigrams);
             Assert.AreEqual(1E6, oneKg.Milligrams);
+
+            Assert.AreEqual(907.18474, oneKg.ShortTons, Delta);
+            Assert.AreEqual(1016.0469088, oneKg.LongTons, Delta);
         }
 
         [Test]
@@ -35,7 +40,10 @@ public void MassUnitsRoundTrip()
             Assert.AreEqual(1, Mass.FromGrams(oneKg.Grams).Kilograms); 
             Assert.AreEqual(1, Mass.FromDecigrams(oneKg.Decigrams).Kilograms); 
             Assert.AreEqual(1, Mass.FromCentigrams(oneKg.Centigrams).Kilograms); 
-            Assert.AreEqual(1, Mass.FromMilligrams(oneKg.Milligrams).Kilograms); 
+            Assert.AreEqual(1, Mass.FromMilligrams(oneKg.Milligrams).Kilograms);
+
+            Assert.AreEqual(1, Mass.FromShortTons(oneKg.ShortTons).Kilograms, Delta);
+            Assert.AreEqual(1, Mass.FromLongTons(oneKg.LongTons).Kilograms, Delta);
         }
     }
 }