Allow adding generic strategies to generic builder (#1386)

Author: martintmk

src/Polly.Core/ResilienceStrategyBuilderExtensions.cs

@@ -25,6 +25,23 @@ public static TBuilder AddStrategy<TBuilder>(this TBuilder builder, ResilienceSt
         return builder.AddStrategy(_ => strategy, EmptyOptions.Instance);
     }
 
+    /// <summary>
+    /// Adds an already created strategy instance to the builder.
+    /// </summary>
+    /// <typeparam name="TResult">The type of the result.</typeparam>
+    /// <param name="builder">The builder instance.</param>
+    /// <param name="strategy">The strategy instance.</param>
+    /// <returns>The same builder instance.</returns>
+    /// <exception cref="ArgumentNullException">Thrown when <paramref name="strategy"/> is null.</exception>
+    /// <exception cref="InvalidOperationException">Thrown when this builder was already used to create a strategy. The builder cannot be modified after it has been used.</exception>
+    public static ResilienceStrategyBuilder<TResult> AddStrategy<TResult>(this ResilienceStrategyBuilder<TResult> builder, ResilienceStrategy<TResult> strategy)
+    {
+        Guard.NotNull(builder);
+        Guard.NotNull(strategy);
+
+        return builder.AddStrategy(strategy.Strategy);
+    }
+
     /// <summary>
     /// Adds a strategy to the builder.
     /// </summary>

test/Polly.Core.Tests/GenericResilienceStrategyBuilderTests.cs

@@ -51,4 +51,19 @@ public void Build_Ok()
         strategy.Should().NotBeNull();
         strategy.Strategy.Should().BeOfType<ResilienceStrategyPipeline>().Subject.Strategies.Should().HaveCount(2);
     }
+
+    [Fact]
+    public void AddGenericStrategy_Ok()
+    {
+        // arrange
+        var testStrategy = new ResilienceStrategy<string>(new TestResilienceStrategy());
+        _builder.AddStrategy(testStrategy);
+
+        // act
+        var strategy = _builder.Build();
+
+        // assert
+        strategy.Should().NotBeNull();
+        strategy.Strategy.Should().Be(testStrategy.Strategy);
+    }
 }