Add BeApproximately for both subject and expected of type double?

Src/FluentAssertions/NumericAssertionsExtensions.cs

@@ -775,6 +775,47 @@ public static class NumericAssertionsExtensions
             return new AndConstraint<NullableNumericAssertions<double>>(parent);
         }
 
+        /// <summary>
+        /// Asserts a double value approximates another value as close as possible.
+        /// Does not throw if both null value approximates another null value.
+        /// </summary>
+        /// <param name="parent">The <see cref="NumericAssertions{T}"/> object that is being extended.</param>
+        /// <param name="expectedValue">
+        /// The expected value to compare the actual value with.
+        /// </param>
+        /// <param name="precision">
+        /// The maximum amount of which the two values may differ.
+        /// </param>
+        /// <param name="because">
+        /// A formatted phrase as is supported by <see cref="string.Format(string,object[])"/> explaining why the assertion
+        /// is needed. If the phrase does not start with the word <i>because</i>, it is prepended automatically.
+        /// </param>
+        /// <param name="becauseArgs">
+        /// Zero or more objects to format using the placeholders in <see cref="because"/>.
+        /// </param>
+        public static AndConstraint<NullableNumericAssertions<double>> BeApproximately(this NullableNumericAssertions<double> parent,
+            double? expectedValue, double precision, string because = "",
+            params object[] becauseArgs)
+        {
+            if(parent.Subject == null && expectedValue == null)
+            {
+                return new AndConstraint<NullableNumericAssertions<double>>(parent);
+            }
+
+            bool succeeded = Execute.Assertion
+                .ForCondition(expectedValue != null)
+                .BecauseOf(because, becauseArgs)
+                .FailWith("Expected {context:value} to approximate {0} +/- {1}{reason}, but it was {2}.", expectedValue, precision, parent.Subject);
+
+            if (succeeded)
+            {
+                // ReSharper disable once PossibleInvalidOperationException
+                parent.BeApproximately(expectedValue.Value, precision, because, becauseArgs);
+            }
+
+            return new AndConstraint<NullableNumericAssertions<double>>(parent);
+        }
+
         /// <summary>
         /// Asserts a double value approximates another value as close as possible.
         /// </summary>

Tests/Shared.Specs/NullableNumericAssertionSpecs.cs

@@ -314,6 +314,108 @@ public void When_nullable_double_is_indeed_approximating_a_value_it_should_not_t
             act.Should().NotThrow();
         }
 
+        [Fact]
+        public void When_nullable_double_is_indeed_approximating_a_nullable_value_it_should_not_throw()
+        {
+            //-----------------------------------------------------------------------------------------------------------
+            // Arrange
+            //-----------------------------------------------------------------------------------------------------------
+            double? value = 3.1415927;
+            double? expected = 3.142;
+
+            //-----------------------------------------------------------------------------------------------------------
+            // Act
+            //-----------------------------------------------------------------------------------------------------------
+            Action act = () => value.Should().BeApproximately(expected, 0.1);
+
+            //-----------------------------------------------------------------------------------------------------------
+            // Assert
+            //-----------------------------------------------------------------------------------------------------------
+            act.Should().NotThrow();
+        }
+
+        [Fact]
+        public void When_nullable_double_is_null_approximating_a_nullable_null_value_it_should_not_throw()
+        {
+            //-----------------------------------------------------------------------------------------------------------
+            // Arrange
+            //-----------------------------------------------------------------------------------------------------------
+            double? value = null;
+            double? expected = null;
+
+            //-----------------------------------------------------------------------------------------------------------
+            // Act
+            //-----------------------------------------------------------------------------------------------------------
+            Action act = () => value.Should().BeApproximately(expected, 0.1);
+
+            //-----------------------------------------------------------------------------------------------------------
+            // Assert
+            //-----------------------------------------------------------------------------------------------------------
+            act.Should().NotThrow();
+        }
+
+        [Fact]
+        public void When_nullable_double_with_value_is_not_approximating_a_non_null_nullable_value_it_should_throw()
+        {
+            //-----------------------------------------------------------------------------------------------------------
+            // Arrange
+            //-----------------------------------------------------------------------------------------------------------
+            double? value = 13;
+            double? expected = 12;
+
+            //-----------------------------------------------------------------------------------------------------------
+            // Act
+            //-----------------------------------------------------------------------------------------------------------
+            Action act = () => value.Should().BeApproximately(expected, 0.1);
+
+            //-----------------------------------------------------------------------------------------------------------
+            // Assert
+            //-----------------------------------------------------------------------------------------------------------
+            act.Should().Throw<XunitException>().WithMessage("Expected*12.0*0.1*13.0*");
+        }
+
+        [Fact]
+        public void When_nullable_double_is_null_approximating_a_non_null_nullable_value_it_should_throw()
+        {
+            //-----------------------------------------------------------------------------------------------------------
+            // Arrange
+            //-----------------------------------------------------------------------------------------------------------
+            double? value = null;
+            double? expected = 12;
+
+            //-----------------------------------------------------------------------------------------------------------
+            // Act
+            //-----------------------------------------------------------------------------------------------------------
+            Action act = () => value.Should().BeApproximately(expected, 0.1);
+
+            //-----------------------------------------------------------------------------------------------------------
+            // Assert
+            //-----------------------------------------------------------------------------------------------------------
+            act.Should().Throw<XunitException>().WithMessage(
+                "Expected value to approximate 12.0 +/- 0.1, but it was <null>.");
+        }
+
+        [Fact]
+        public void When_nullable_double_is_not_null_approximating_a_null_value_it_should_throw()
+        {
+            //-----------------------------------------------------------------------------------------------------------
+            // Arrange
+            //-----------------------------------------------------------------------------------------------------------
+            double? value = 12;
+            double? expected = null;
+
+            //-----------------------------------------------------------------------------------------------------------
+            // Act
+            //-----------------------------------------------------------------------------------------------------------
+            Action act = () => value.Should().BeApproximately(expected, 0.1);
+
+            //-----------------------------------------------------------------------------------------------------------
+            // Assert
+            //-----------------------------------------------------------------------------------------------------------
+            act.Should().Throw<XunitException>().WithMessage(
+                "Expected value to approximate <null> +/- 0.1, but it was 12.0.");
+        }
+
         [Fact]
         public void When_nullable_double_has_no_value_it_should_throw()
         {