Fix DependencyContext splitting on semi-colon (#87518)

* Fix DependencyContext splitting on semi-colon

5e67657 introduced a bug in DependencyContextPaths where the static array is not initialized before it is being used in the Create static method.

This fix removes the static array since it is only used once.

- Don't cache the semicolon array since it is only used once at startup
- Skip test on netfx since it doesn't work.

src/libraries/Microsoft.Extensions.DependencyModel/src/DependencyContextPaths.cs

@@ -20,8 +20,6 @@ internal sealed class DependencyContextPaths
 
         public IEnumerable<string> NonApplicationPaths { get; }
 
-        private static readonly char[] s_semicolon = new[] { ';' };
-
         public DependencyContextPaths(
             string? application,
             string? sharedRuntime,
@@ -42,7 +40,13 @@ private static DependencyContextPaths GetCurrent()
 
         internal static DependencyContextPaths Create(string? depsFiles, string? sharedRuntime)
         {
-            string[]? files = depsFiles?.Split(s_semicolon, StringSplitOptions.RemoveEmptyEntries);
+#if NETCOREAPP
+            const char separator = ';';
+#else
+            // This method is only executed once at startup. No need to cache the char[].
+            char[] separator = { ';' };
+#endif
+            string[]? files = depsFiles?.Split(separator, StringSplitOptions.RemoveEmptyEntries);
             string? application = files != null && files.Length > 0 ? files[0] : null;
 
             string[]? nonApplicationPaths = files?

src/libraries/Microsoft.Extensions.DependencyModel/tests/DependencyContextTests.cs

@@ -276,6 +276,19 @@ public void MergeMergesRuntimeGraph()
                 Subject.Fallbacks.Should().BeEquivalentTo("win7-x64", "win7-x86");
         }
 
+        [Fact]
+        [SkipOnTargetFramework(TargetFrameworkMonikers.NetFramework, "GetEntryAssembly() returns null")]
+        public void DefaultWorksCorrectly()
+        {
+            // only need to assert the context contains non-null properties.
+
+            var context = DependencyContext.Default;
+            Assert.NotNull(context);
+            Assert.NotNull(context.RuntimeGraph);
+            Assert.NotNull(context.RuntimeLibraries);
+            Assert.NotNull(context.Target);
+        }
+
         private TargetInfo CreateTargetInfo()
         {
             return new TargetInfo(