Added check and argument exception for get to property with no value

Author: robdmoore

NTestDataBuilder.Tests/GetSetTests.cs

@@ -1,4 +1,5 @@
-using NTestDataBuilder.Tests.Builders;
+using System;
+using NTestDataBuilder.Tests.Builders;
 using NUnit.Framework;
 
 namespace NTestDataBuilder.Tests
@@ -56,5 +57,14 @@ public void GivenAValueHasBeenSetAgainstTwoProperties_WhenRetrievingTheValueForT
 
             Assert.That(retrieved, Is.EqualTo(SetValue));
         }
+
+        [Test]
+        public void GivenNoValueHasBeenSetForAProperty_WhenRetrievingTheValueForThatProperty_ThenThrowAnException()
+        {
+            var ex = Assert.Throws<ArgumentException>(() => _b.Get(x => x.FirstName));
+
+            Assert.That(ex.Message, Is.StringStarting("No value has been recorded yet for FirstName; consider using Has(x => x.FirstName) to check for a value first."));
+            Assert.That(ex.ParamName, Is.EqualTo("property"));
+        }
     }
 }

NTestDataBuilder/DataBuilder.cs

@@ -53,9 +53,29 @@ public void Set<TValue>(Expression<Func<TEntity, TValue>> property, TValue value
         /// <returns>The recorded value of the property</returns>
         public TValue Get<TValue>(Expression<Func<TEntity, TValue>> property)
         {
+            if (!Has(property))
+                throw new ArgumentException(
+                    string.Format(
+                        "No value has been recorded yet for {0}; consider using Has(x => x.{0}) to check for a value first.",
+                        GetPropertyName(property)
+                    ),
+                    "property"
+                );
+
             return (TValue)_properties[GetPropertyName(property)];
         }
 
+        /// <summary>
+        /// Returns whether or not there is currently a value recorded against the given property from <see cref="TEntity"/>.
+        /// </summary>
+        /// <typeparam name="TValue">The type of the property</typeparam>
+        /// <param name="property">A lambda expression specifying the property to retrieve the recorded value for</param>
+        /// <returns>Whether or not there is a recorded value for the property</returns>
+        protected bool Has<TValue>(Expression<Func<TEntity, TValue>> property)
+        {
+            return _properties.ContainsKey(GetPropertyName(property));
+        }
+
         private static string GetPropertyName<TValue>(Expression<Func<TEntity, TValue>> property)
         {
             var memExp = property.Body as MemberExpression;