Should().Throw and Should().NotThrow throw for Func<T> (#951)

Src/FluentAssertions/AssertionExtensions.cs

@@ -634,14 +634,24 @@ public static ActionAssertions Should(this Action action)
 
         /// <summary>
         /// Returns a <see cref="AsyncFunctionAssertions"/> object that can be used to assert the
-        /// current <see cref="System.Func{T}"/> .
+        /// current <see cref="System.Func{Task}"/> .
         /// </summary>
         [Pure]
         public static AsyncFunctionAssertions Should(this Func<Task> action)
         {
             return new AsyncFunctionAssertions(action, extractor);
         }
 
+        /// <summary>
+        /// Returns a <see cref="FunctionAssertions{T}"/> object that can be used to assert the
+        /// current <see cref="System.Func{T}"/> .
+        /// </summary>
+        [Pure]
+        public static FunctionAssertions<T> Should<T>(this Func<T> func)
+        {
+            return new FunctionAssertions<T>(func, extractor);
+        }
+
 #if NET45 || NET47 || NETCOREAPP2_0
 
         /// <summary>

Src/FluentAssertions/Specialized/FunctionAssertions.cs

@@ -0,0 +1,175 @@
+using System;
+using System.Diagnostics;
+using System.Linq;
+
+using FluentAssertions.Execution;
+using FluentAssertions.Primitives;
+
+namespace FluentAssertions.Specialized
+{
+    /// <summary>
+    /// Contains a number of methods to assert that a synchronous function yields the expected result.
+    /// </summary>
+    [DebuggerNonUserCode]
+    public class FunctionAssertions<T> : ReferenceTypeAssertions<Func<T>, FunctionAssertions<T>>
+    {
+        private readonly IExtractExceptions extractor;
+
+        public FunctionAssertions(Func<T> subject, IExtractExceptions extractor)
+        {
+            this.extractor = extractor;
+            Subject = subject;
+        }
+
+        protected override string Identifier => "function";
+
+        /// <summary>
+        /// Asserts that the current <see cref="Func{T}"/> throws an exception of type <typeparamref name="TException"/>.
+        /// </summary>
+        /// <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 ExceptionAssertions<TException> Throw<TException>(string because = "", params object[] becauseArgs)
+            where TException : Exception
+        {
+            Exception exception = InvokeSubjectWithInterception();
+            return Throw<TException>(exception, because, becauseArgs);
+        }
+
+        /// <summary>
+        /// Asserts that the current <see cref="Func{T}"/> throws an exception of the exact type <typeparamref name="TException"/> (and not a derived exception type).
+        /// </summary>
+        /// <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 ExceptionAssertions<TException> ThrowExactly<TException>(string because = "", params object[] becauseArgs)
+            where TException : Exception
+        {
+            var exceptionAssertions = Throw<TException>(because, becauseArgs);
+            exceptionAssertions.Which.GetType().Should().Be<TException>(because, becauseArgs);
+            return exceptionAssertions;
+        }
+
+        /// <summary>
+        /// Asserts that the current <see cref="Func{T}"/> does not throw any exception.
+        /// </summary>
+        /// <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 AndWhichConstraint<FunctionAssertions<T>, T> NotThrow(string because = "", params object[] becauseArgs)
+        {
+            try
+            {
+                T result = Subject();
+                return new AndWhichConstraint<FunctionAssertions<T>, T>(this, result);
+            }
+            catch (Exception exception)
+            {
+                NotThrow(exception, because, becauseArgs);
+                return null;
+
+            }
+        }
+
+        /// <summary>
+        /// Asserts that the current <see cref="Func{T}"/> does not throw an exception of type <typeparamref name="TException"/>.
+        /// </summary>
+        /// <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 void NotThrow<TException>(string because = "", params object[] becauseArgs)
+            where TException : Exception
+        {
+            try
+            {
+                Subject();
+            }
+            catch (Exception exception)
+            {
+                NotThrow<TException>(exception, because, becauseArgs);
+            }
+        }
+
+        private static void NotThrow(Exception exception, string because, object[] becauseArgs)
+        {
+            Exception nonAggregateException = GetFirstNonAggregateException(exception);
+
+            Execute.Assertion
+                .BecauseOf(because, becauseArgs)
+                .FailWith("Did not expect any exception{reason}, but found a {0} with message {1}.",
+                    nonAggregateException.GetType(), nonAggregateException.ToString());
+        }
+
+        private static void NotThrow<TException>(Exception exception, string because, object[] becauseArgs) where TException : Exception
+        {
+            Exception nonAggregateException = GetFirstNonAggregateException(exception);
+
+            if (nonAggregateException != null)
+            {
+                Execute.Assertion
+                    .ForCondition(!(nonAggregateException is TException))
+                    .BecauseOf(because, becauseArgs)
+                    .FailWith("Did not expect {0}{reason}, but found one with message {1}.",
+                        typeof(TException), nonAggregateException.ToString());
+            }
+        }
+
+        private static Exception GetFirstNonAggregateException(Exception exception)
+        {
+            Exception nonAggregateException = exception;
+            while (nonAggregateException is AggregateException)
+            {
+                nonAggregateException = nonAggregateException.InnerException;
+            }
+
+            return nonAggregateException;
+        }
+
+        private ExceptionAssertions<TException> Throw<TException>(Exception exception, string because, object[] becauseArgs)
+            where TException : Exception
+        {
+            Execute.Assertion
+                .ForCondition(exception != null)
+                .BecauseOf(because, becauseArgs)
+                .FailWith("Expected {0}{reason}, but no exception was thrown.", typeof(TException));
+
+            var exceptions = extractor.OfType<TException>(exception);
+
+            Execute.Assertion
+                .ForCondition(exceptions.Any())
+                .BecauseOf(because, becauseArgs)
+                .FailWith("Expected {0}{reason}, but found {1}.", typeof(TException), exception);
+
+            return new ExceptionAssertions<TException>(exceptions);
+        }
+
+        private Exception InvokeSubjectWithInterception()
+        {
+            try
+            {
+                Subject();
+                return null;
+            }
+            catch (Exception exception)
+            {
+                return exception;
+            }
+        }
+    }
+}