diff --git a/src/Numerics/Numerics.csproj b/src/Numerics/Numerics.csproj
index 00b170d5..d2317da4 100644
--- a/src/Numerics/Numerics.csproj
+++ b/src/Numerics/Numerics.csproj
@@ -58,7 +58,7 @@
     <ErrorReport>prompt</ErrorReport>
     <WarningLevel>4</WarningLevel>
     <CodeAnalysisRuleSet>AllRules.ruleset</CodeAnalysisRuleSet>
-    <LangVersion>5</LangVersion>
+    <LangVersion>6</LangVersion>
   </PropertyGroup>
   <PropertyGroup Condition="'$(Configuration)|$(Platform)' == 'Release-Signed|AnyCPU'">
     <OutputPath>..\..\out\lib-signed\Net40\</OutputPath>
@@ -115,8 +115,6 @@
     <Compile Include="LinearRegression\Options.cs" />
     <Compile Include="OdeSolvers\AdamsBashforth.cs" />
     <Compile Include="OdeSolvers\RungeKutta.cs" />
-    <Compile Include="Optimization\BaseEvaluation.cs" />
-    <Compile Include="Optimization\BaseObjectiveFunction.cs" />
     <Compile Include="Optimization\BfgsMinimizerBase.cs" />
     <Compile Include="Optimization\LineSearch\WolfeLineSearch.cs" />
     <Compile Include="Optimization\NelderMeadSimplex.cs" />
@@ -259,8 +257,6 @@
     <Compile Include="RootFinding\NewtonRaphson.cs" />
     <Compile Include="RootFinding\RobustNewtonRaphson.cs" />
     <Compile Include="RootFinding\Secant.cs" />
-    <Compile Include="RootFinding\WolfeRule.cs" />
-    <Compile Include="Optimization\LineSearch\WolfeRule.cs" />
     <Compile Include="RootFinding\ZeroCrossingBracketing.cs" />
     <Compile Include="RootFinding\Brent.cs" />
     <Compile Include="FindRoots.cs" />
@@ -519,6 +515,5 @@
       <LastGenOutput>Resources.Designer.cs</LastGenOutput>
     </EmbeddedResource>
   </ItemGroup>
-  <ItemGroup />
   <Import Project="$(MSBuildToolsPath)\Microsoft.CSharp.targets" />
 </Project>
\ No newline at end of file
diff --git a/src/UnitTests/OptimizationTests/BfgsBMinimizerTests.cs b/src/UnitTests/OptimizationTests/BfgsBMinimizerTests.cs
index 33125613..19ff2773 100644
--- a/src/UnitTests/OptimizationTests/BfgsBMinimizerTests.cs
+++ b/src/UnitTests/OptimizationTests/BfgsBMinimizerTests.cs
@@ -38,6 +38,7 @@ using System.Collections.Generic;
 using MathNet.Numerics.UnitTests.OptimizationTests.TestFunctions;
 using System.Collections;
 using MathNet.Numerics.Optimization.ObjectiveFunctions;
+using NUnit.Framework.Interfaces;
 
 namespace MathNet.Numerics.UnitTests.OptimizationTests
 {
diff --git a/src/UnitTests/OptimizationTests/BfgsMinimizerTests.cs b/src/UnitTests/OptimizationTests/BfgsMinimizerTests.cs
index be4e4b97..8622886f 100644
--- a/src/UnitTests/OptimizationTests/BfgsMinimizerTests.cs
+++ b/src/UnitTests/OptimizationTests/BfgsMinimizerTests.cs
@@ -7,6 +7,7 @@ using System.Text;
 using MathNet.Numerics.UnitTests.OptimizationTests.TestFunctions;
 using System.Collections.Generic;
 using System.Collections;
+using NUnit.Framework.Interfaces;
 
 namespace MathNet.Numerics.UnitTests.OptimizationTests
 {
diff --git a/src/UnitTests/OptimizationTests/ConjugateGradientMinimizerTests.cs b/src/UnitTests/OptimizationTests/ConjugateGradientMinimizerTests.cs
index e911b1cc..254a1db7 100644
--- a/src/UnitTests/OptimizationTests/ConjugateGradientMinimizerTests.cs
+++ b/src/UnitTests/OptimizationTests/ConjugateGradientMinimizerTests.cs
@@ -6,6 +6,7 @@ using MathNet.Numerics.UnitTests.OptimizationTests.TestFunctions;
 using System.Collections;
 using System.Collections.Generic;
 using System.Linq;
+using NUnit.Framework.Interfaces;
 
 namespace MathNet.Numerics.UnitTests.OptimizationTests
 {
diff --git a/src/UnitTests/OptimizationTests/NelderMeadSimplexTests.cs b/src/UnitTests/OptimizationTests/NelderMeadSimplexTests.cs
index b01bfc32..ef16d76f 100644
--- a/src/UnitTests/OptimizationTests/NelderMeadSimplexTests.cs
+++ b/src/UnitTests/OptimizationTests/NelderMeadSimplexTests.cs
@@ -36,6 +36,7 @@ using System;
 using System.Collections;
 using System.Collections.Generic;
 using System.Linq;
+using NUnit.Framework.Interfaces;
 
 namespace MathNet.Numerics.UnitTests.OptimizationTests
 {
diff --git a/src/UnitTests/OptimizationTests/NewtonMinimizerTests.cs b/src/UnitTests/OptimizationTests/NewtonMinimizerTests.cs
index 735af225..6f66c2c7 100644
--- a/src/UnitTests/OptimizationTests/NewtonMinimizerTests.cs
+++ b/src/UnitTests/OptimizationTests/NewtonMinimizerTests.cs
@@ -7,6 +7,7 @@ using MathNet.Numerics.UnitTests.OptimizationTests.TestFunctions;
 using System.Collections.Generic;
 using System.Collections;
 using System.Linq;
+using NUnit.Framework.Interfaces;
 
 namespace MathNet.Numerics.UnitTests.OptimizationTests
 {
diff --git a/src/UnitTests/UnitTests.csproj b/src/UnitTests/UnitTests.csproj
index 7a30ea3c..35580c2a 100644
--- a/src/UnitTests/UnitTests.csproj
+++ b/src/UnitTests/UnitTests.csproj
@@ -46,7 +46,7 @@
     <PlatformTarget>AnyCPU</PlatformTarget>
     <NoWarn>1591</NoWarn>
     <Prefer32Bit>false</Prefer32Bit>
-    <LangVersion>5</LangVersion>
+    <LangVersion>6</LangVersion>
   </PropertyGroup>
   <PropertyGroup Condition="'$(Configuration)|$(Platform)' == 'Release-Signed|AnyCPU'">
     <OutputPath>..\..\out\test-signed\Net40\</OutputPath>
@@ -366,8 +366,6 @@
     <Compile Include="LinearAlgebraTests\VectorToStringTests.cs" />
     <Compile Include="OdeSolvers\OdeSolverTest.cs" />
     <Compile Include="Random\RandomSerializationTests.cs" />
-    <Compile Include="OptimizationTests\RosenbrockFunction.cs" />
-    <Compile Include="OptimizationTests\TestNewtonMinimizer.cs" />
     <Compile Include="OptimizationTests\TestCaseDataExtensions.cs" />
     <Compile Include="OptimizationTests\TestFunctions\BrownAndDennisFunction.cs" />
     <Compile Include="OptimizationTests\TestFunctions\HelicalValleyFunction.cs" />