diff --git a/X10D.Tests/X10D.Tests.csproj b/X10D.Tests/X10D.Tests.csproj
index f6f97d4..6bba44b 100644
--- a/X10D.Tests/X10D.Tests.csproj
+++ b/X10D.Tests/X10D.Tests.csproj
@@ -1,7 +1,8 @@
 <Project Sdk="Microsoft.NET.Sdk">
 
     <PropertyGroup>
-        <TargetFramework>net6.0</TargetFramework>
+        <TargetFrameworks>netstandard2.1;net6.0</TargetFrameworks>
+        <LangVersion>10.0</LangVersion>
         <IsPackable>false</IsPackable>
         <Nullable>enable</Nullable>
         <ImplicitUsings>true</ImplicitUsings>
diff --git a/X10D.Tests/src/Collections/CollectionTests.cs b/X10D.Tests/src/Collections/CollectionTests.cs
index 850ec06..835a6d6 100644
--- a/X10D.Tests/src/Collections/CollectionTests.cs
+++ b/X10D.Tests/src/Collections/CollectionTests.cs
@@ -67,10 +67,11 @@ public class CollectionTests
             Assert.IsTrue(IsDisposed = true);
         }
 
-        public ValueTask DisposeAsync()
+#pragma warning disable CS1998
+        public async ValueTask DisposeAsync()
+#pragma warning restore CS1998
         {
             Assert.IsTrue(IsDisposed = true);
-            return ValueTask.CompletedTask;
         }
     }
 }
diff --git a/X10D.Tests/src/Collections/EnumerableTests.cs b/X10D.Tests/src/Collections/EnumerableTests.cs
index 851277f..533a00e 100644
--- a/X10D.Tests/src/Collections/EnumerableTests.cs
+++ b/X10D.Tests/src/Collections/EnumerableTests.cs
@@ -126,10 +126,11 @@ public class EnumerableTests
             Assert.IsTrue(IsDisposed = true);
         }
 
-        public ValueTask DisposeAsync()
+#pragma warning disable CS1998
+        public async ValueTask DisposeAsync()
+#pragma warning restore CS1998
         {
             Assert.IsTrue(IsDisposed = true);
-            return ValueTask.CompletedTask;
         }
     }
 }
diff --git a/X10D.Tests/src/Math/DecimalTests.cs b/X10D.Tests/src/Math/DecimalTests.cs
index a8ce150..b84b1cf 100644
--- a/X10D.Tests/src/Math/DecimalTests.cs
+++ b/X10D.Tests/src/Math/DecimalTests.cs
@@ -7,6 +7,7 @@ namespace X10D.Tests.Math;
 [TestClass]
 public class DecimalTests
 {
+#if NETCOREAPP3_0_OR_GREATER
     [TestMethod]
     public void ComplexSqrt_ShouldBeCorrect_GivenReal()
     {
@@ -25,6 +26,7 @@ public class DecimalTests
         Assert.AreEqual(new Complex(0, 3.0), (-9.0m).ComplexSqrt());
         Assert.AreEqual(new Complex(0, 4.0), (-16.0m).ComplexSqrt());
     }
+#endif
 
     [TestMethod]
     public void IsEven_ShouldBeFalse_GivenOddNumber()
diff --git a/X10D.Tests/src/Math/DoubleTests.cs b/X10D.Tests/src/Math/DoubleTests.cs
index 2496348..96034bc 100644
--- a/X10D.Tests/src/Math/DoubleTests.cs
+++ b/X10D.Tests/src/Math/DoubleTests.cs
@@ -29,6 +29,7 @@ public class DoubleTests
         Assert.AreEqual(12.0, 0.20943951023931953.RadiansToDegrees(), 1e-6);
     }
 
+#if NETCOREAPP3_0_OR_GREATER
     [TestMethod]
     public void ComplexSqrt_ShouldBeCorrect_GivenReal()
     {
@@ -60,6 +61,7 @@ public class DoubleTests
     {
         Assert.AreEqual(Complex.NaN, double.NaN.ComplexSqrt());
     }
+#endif
 
     [TestMethod]
     public void IsEven_ShouldBeFalse_GivenOddNumber()
diff --git a/X10D.Tests/src/Math/SingleTests.cs b/X10D.Tests/src/Math/SingleTests.cs
index 12b1ae2..82c2c0d 100644
--- a/X10D.Tests/src/Math/SingleTests.cs
+++ b/X10D.Tests/src/Math/SingleTests.cs
@@ -29,6 +29,7 @@ public class SingleTests
         Assert.AreEqual(12.0f, 0.20943952f.RadiansToDegrees(), 1e-6f);
     }
 
+#if NETCOREAPP3_0_OR_GREATER
     [TestMethod]
     public void ComplexSqrt_ShouldBeCorrect_GivenReal()
     {
@@ -60,6 +61,7 @@ public class SingleTests
     {
         Assert.AreEqual(Complex.NaN, float.NaN.ComplexSqrt());
     }
+#endif
 
     [TestMethod]
     public void IsEven_ShouldBeFalse_GivenOddNumber()
diff --git a/X10D.Tests/src/Text/CoreTests.cs b/X10D.Tests/src/Text/CoreTests.cs
index f6d9aea..6c4c58a 100644
--- a/X10D.Tests/src/Text/CoreTests.cs
+++ b/X10D.Tests/src/Text/CoreTests.cs
@@ -6,6 +6,7 @@ namespace X10D.Tests.Text;
 [TestClass]
 public class CoreTests
 {
+#if NET5_0_OR_GREATER
     [TestMethod]
     public void ToJsonShouldNotBeEmpty()
     {
@@ -23,4 +24,5 @@ public class CoreTests
         CollectionAssert.AreEqual(source, target);
         CollectionAssert.AreEquivalent(source, target);
     }
+#endif
 }
diff --git a/X10D.Tests/src/Text/RuneTests.cs b/X10D.Tests/src/Text/RuneTests.cs
index 7d3315d..8edfa9b 100644
--- a/X10D.Tests/src/Text/RuneTests.cs
+++ b/X10D.Tests/src/Text/RuneTests.cs
@@ -1,4 +1,5 @@
-using System.Text;
+#if NETCOREAPP3_0_OR_GREATER
+using System.Text;
 using Microsoft.VisualStudio.TestTools.UnitTesting;
 using X10D.Text;
 
@@ -37,3 +38,4 @@ public class RuneTests
         Assert.ThrowsException<ArgumentOutOfRangeException>(() => new Rune('a').Repeat(-1));
     }
 }
+#endif
diff --git a/X10D.Tests/src/Text/StringTests.cs b/X10D.Tests/src/Text/StringTests.cs
index 5d509f0..852bec2 100644
--- a/X10D.Tests/src/Text/StringTests.cs
+++ b/X10D.Tests/src/Text/StringTests.cs
@@ -1,5 +1,7 @@
-using System.Text;
+using System.Text;
+#if NET5_0_OR_GREATER
 using System.Text.Json.Serialization;
+#endif
 using Microsoft.VisualStudio.TestTools.UnitTesting;
 using X10D.Text;
 
@@ -149,6 +151,7 @@ public class StringTests
         Assert.ThrowsException<ArgumentException>(() => " ".EnumParse<DayOfWeek>());
     }
 
+#if NET5_0_OR_GREATER
     [TestMethod]
     public void FromJson_ShouldDeserializeCorrectly_GivenJsonString()
     {
@@ -162,6 +165,7 @@ public class StringTests
         Assert.AreEqual(2, target.Values[1]);
         Assert.AreEqual(3, target.Values[2]);
     }
+#endif
 
     [TestMethod]
     public void GetBytes_ShouldReturnUtf8Bytes_GivenHelloWorld()
@@ -441,9 +445,11 @@ public class StringTests
         Assert.AreEqual(alternative, ((string?)null).WithWhiteSpaceAlternative(alternative));
     }
 
+#if NET5_0_OR_GREATER
     private struct SampleStructure
     {
         [JsonPropertyName("values")]
         public int[] Values { get; set; }
     }
+#endif
 }
diff --git a/X10D.Tests/src/Time/DoubleTests.cs b/X10D.Tests/src/Time/DoubleTests.cs
index 17c7f2e..799c1b1 100644
--- a/X10D.Tests/src/Time/DoubleTests.cs
+++ b/X10D.Tests/src/Time/DoubleTests.cs
@@ -1,4 +1,5 @@
-using Microsoft.VisualStudio.TestTools.UnitTesting;
+#if NET5_0_OR_GREATER
+using Microsoft.VisualStudio.TestTools.UnitTesting;
 using X10D.Time;
 
 namespace X10D.Tests.Time;
@@ -51,3 +52,4 @@ public class DoubleTests
         Assert.IsTrue((_negativeOne).Weeks() < TimeSpan.Zero);
     }
 }
+#endif