added .NET Standard 2.0 compatibility

src/Serilog.Expressions/Expressions/Helpers.cs

@@ -0,0 +1,44 @@
+// Copyright Serilog Contributors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#if NETSTANDARD2_0
+
+using System;
+
+namespace Serilog.Expressions
+{
+    /// <summary>
+    /// Helper methods.
+    /// </summary>
+    internal static class Helpers
+    {
+        /// <summary>
+        /// Backport .NET Standard 2.1 additions to maintain .NET Standard 2.0 compatibility.
+        /// Returns a value indicating whether a specified string occurs within this string, using the specified comparison rules.
+        /// 
+        /// From;
+        /// https://github.com/dotnet/runtime/issues/22198
+        /// https://stackoverflow.com/questions/444798/case-insensitive-containsstring/444818#444818
+        /// </summary>
+        /// <param name="source">input string</param>
+        /// <param name="value">The string to seek.</param>
+        /// <param name="comparisonType">Specifies the rule to use in the comparison.</param>
+        /// <returns></returns>
+        public static bool Contains(this string source, string value, StringComparison comparisonType)
+        {
+            return source?.IndexOf(value, comparisonType) >= 0;
+        }
+    }
+}
+#endif

src/Serilog.Expressions/Serilog.Expressions.csproj

@@ -5,7 +5,7 @@
       events, ideal for use with JSON or XML configuration.</Description>
     <VersionPrefix>1.1.0</VersionPrefix>
     <Authors>Serilog Contributors</Authors>
-    <TargetFramework>netstandard2.1</TargetFramework>
+    <TargetFrameworks>netstandard2.0;netstandard2.1</TargetFrameworks>
     <TreatWarningsAsErrors>true</TreatWarningsAsErrors>
     <GenerateDocumentationFile>true</GenerateDocumentationFile>
     <RootNamespace>Serilog</RootNamespace>

src/Serilog.Expressions/System.Diagnostics.CodeAnalysis/NotNullAttributes.cs

@@ -0,0 +1,26 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+
+#if NETSTANDARD2_0
+//From: https://medium.com/@SergioPedri/enabling-and-using-c-9-features-on-older-and-unsupported-runtimes-ce384d8debb
+namespace System.Diagnostics.CodeAnalysis
+{
+    /// <summary>Specifies that an output will not be null even if the corresponding type allows it. Specifies that an input argument was not null when the call returns.</summary>
+    [AttributeUsage(AttributeTargets.Field | AttributeTargets.Parameter | AttributeTargets.Property | AttributeTargets.ReturnValue, Inherited = false)]
+    internal sealed class NotNullAttribute : Attribute { }
+
+    /// <summary>Specifies that when a method returns <see cref="ReturnValue"/>, the parameter may be null even if the corresponding type disallows it.</summary>
+    [AttributeUsage(AttributeTargets.Parameter, Inherited = false)]
+    internal sealed class MaybeNullWhenAttribute : Attribute
+    {
+        /// <summary>Initializes the attribute with the specified return value condition.</summary>
+        /// <param name="returnValue">
+        /// The return value condition. If the method returns this value, the associated parameter may be null.
+        /// </param>
+        public MaybeNullWhenAttribute(bool returnValue) => ReturnValue = returnValue;
+
+        /// <summary>Gets the return value condition.</summary>
+        public bool ReturnValue { get; }
+    }
+}
+#endif