Fix #41. (#42)

* Fix #41. Add a method to set GeometryServiceProvider.Instance if nobody else has yet.

Author: airbreather

src/GeoAPI/GeometryServiceProvider.cs

@@ -1,7 +1,6 @@
 using System;
 #if COMPAT_BOOTSTRAP_USING_REFLECTION && HAS_SYSTEM_APPDOMAIN_GETASSEMBLIES && HAS_SYSTEM_REFLECTION_ASSEMBLY_GETEXPORTEDTYPES
 using System.Reflection;
-using System.Threading;
 #endif
 
 namespace GeoAPI
@@ -12,22 +11,88 @@ namespace GeoAPI
  public static class GeometryServiceProvider
  {
  private static volatile IGeometryServices s_instance;
- private static readonly object s_lock = new object();
+
+ /// <summary>
+ /// Make sure only one thread runs <see cref="InitializeInstance"/> at a time.
+ /// </summary>
+ private static readonly object s_autoInitLock = new object();
+
+ /// <summary>
+ /// Make sure that anyone who directly sets <see cref="Instance"/>, including the automatic
+ /// initializer, behaves consistently, regarding <see cref="s_instanceSetDirectly"/> and the
+ /// semantics of <see cref="SetInstanceIfNotAlreadySetDirectly"/>.
+ /// </summary>
+ private static readonly object s_explicitInitLock = new object();
+
+ /// <summary>
+ /// Indicates whether or not <see cref="s_instance"/> has been set directly (i.e., outside
+ /// of the reflection-based initializer).
+ /// </summary>
+ private static bool s_instanceSetDirectly = false;
 
  /// <summary>
  /// Gets or sets the <see cref="IGeometryServices"/> instance.
  /// </summary>
  public static IGeometryServices Instance
  {
  get => s_instance ?? InitializeInstance();
- set => s_instance = value ?? throw new ArgumentNullException(nameof(value));
+ set
+ {
+ if (value == null)
+ {
+ throw new ArgumentNullException(nameof(value));
+ }
+
+ lock (s_explicitInitLock)
+ {
+ s_instance = value;
+ s_instanceSetDirectly = true;
+ }
+ }
+ }
+
+ /// <summary>
+ /// Sets <see cref="Instance"/> to the given value, unless it has already been set directly.
+ /// Both this method and the property's setter itself count as setting it "directly".
+ /// </summary>
+ /// <param name="instance">
+ /// The new value to put into <see cref="Instance"/> if it hasn't already been set directly.
+ /// </param>
+ /// <returns>
+ /// <c>true</c> if <see cref="Instance"/> was set, <c>false</c> otherwise.
+ /// </returns>
+ /// <exception cref="ArgumentNullException">
+ /// Thrown when <paramref name="instance"/> is <see langword="null"/>.
+ /// </exception>
+ public static bool SetInstanceIfNotAlreadySetDirectly(IGeometryServices instance)
+ {
+ if (instance == null)
+ {
+ throw new ArgumentNullException(nameof(instance));
+ }
+
+ lock (s_explicitInitLock)
+ {
+ if (s_instanceSetDirectly)
+ {
+ // someone has already set the value directly before.
+ return false;
+ }
+
+ s_instance = instance;
+
+ // calling this method counts.
+ s_instanceSetDirectly = true;
+ return true;
+ }
  }
 
  private static IGeometryServices InitializeInstance()
  {
 #if COMPAT_BOOTSTRAP_USING_REFLECTION && HAS_SYSTEM_APPDOMAIN_GETASSEMBLIES && HAS_SYSTEM_REFLECTION_ASSEMBLY_GETEXPORTEDTYPES
- lock (s_lock)
+ lock (s_autoInitLock)
  {
+ // see if someone has already set it while we were waiting for the lock.
  var instance = s_instance;
  if (instance != null) return instance;
  foreach (var assembly in AppDomain.CurrentDomain.GetAssemblies())
@@ -64,8 +129,16 @@ private static IGeometryServices InitializeInstance()
  foreach (var constructor in type.GetConstructors())
  if (constructor.IsPublic && constructor.GetParameters().Length == 0)
  {
- Interlocked.CompareExchange(ref s_instance, (IGeometryServices)Activator.CreateInstance(type), null);
- return s_instance;
+ instance = (IGeometryServices)Activator.CreateInstance(type);
+ lock (s_explicitInitLock)
+ {
+ if (!s_instanceSetDirectly)
+ {
+ s_instance = instance;
+ }
+
+ return s_instance;
+ }
  }
  }
  }