Tweaks.

src/OpenTelemetry/Trace/TracerProviderBuilderBase.cs

@@ -19,6 +19,7 @@
 using System;
 using System.Collections.Generic;
 using System.Diagnostics;
+using System.Linq;
 using Microsoft.Extensions.DependencyInjection;
 using OpenTelemetry.Internal;
 using OpenTelemetry.Resources;
@@ -284,15 +285,21 @@ protected virtual TracerProvider OnBuild()
 
             // Step 3: Look for any samplers registered.
 
-            var registeredSampler = serviceProvider.GetService<Sampler>();
+            var registeredSamplers = serviceProvider.GetServices<Sampler>();
+            int registeredSamplerCount = registeredSamplers.Count();
+            if (registeredSamplerCount > 1)
+            {
+                throw new NotSupportedException("Multiple samplers registered in application services is not supported.");
+            }
+
             var sampler = this.sampler;
             if (sampler == null)
             {
-                sampler = registeredSampler ?? new ParentBasedSampler(new AlwaysOnSampler());
+                sampler = registeredSamplers.First() ?? new ParentBasedSampler(new AlwaysOnSampler());
             }
-            else if (registeredSampler != null)
+            else if (registeredSamplerCount > 0)
             {
-                throw new NotSupportedException("A sampler was registered in application services and set on tracer builder directly.");
+                throw new NotSupportedException("Setting sampler directly on tracer builder and through application services is not supported.");
             }
 
             // Step 4: Look for any processors registered.

src/OpenTelemetry/Trace/TracerProviderBuilderExtensions.cs

@@ -19,6 +19,7 @@
 using System;
 using System.Diagnostics;
 using Microsoft.Extensions.DependencyInjection;
+using Microsoft.Extensions.DependencyInjection.Extensions;
 using OpenTelemetry.Internal;
 using OpenTelemetry.Resources;
 
@@ -65,15 +66,18 @@ public static TracerProviderBuilder SetSampler(this TracerProviderBuilder tracer
         /// <summary>
         /// Sets the sampler on the provider.
         /// </summary>
+        /// <remarks>
+        /// Note: The type specified by <typeparamref name="T"/> will be
+        /// registered as a singleton service into application services.
+        /// </remarks>
         /// <typeparam name="T">Sampler type.</typeparam>
         /// <param name="tracerProviderBuilder"><see cref="TracerProviderBuilder"/>.</param>
         /// <returns>The supplied <see cref="TracerProviderBuilder"/> for chaining.</returns>
         public static TracerProviderBuilder SetSampler<T>(this TracerProviderBuilder tracerProviderBuilder)
             where T : Sampler
         {
-            return ConfigureBuilder(
-                tracerProviderBuilder,
-                (sp, builder) => builder.SetSampler(sp.GetRequiredService<T>()));
+            return tracerProviderBuilder
+                .ConfigureServices(services => services.TryAddSingleton<Sampler, T>());
         }
 
         /// <summary>
@@ -139,15 +143,18 @@ public static TracerProviderBuilder AddProcessor(this TracerProviderBuilder trac
         /// <summary>
         /// Adds a processor to the provider which will be retrieved using dependency injection.
         /// </summary>
+        /// <remarks>
+        /// Note: The type specified by <typeparamref name="T"/> will be
+        /// registered as a singleton service into application services.
+        /// </remarks>
         /// <typeparam name="T">Processor type.</typeparam>
         /// <param name="tracerProviderBuilder">TracerProviderBuilder instance.</param>
         /// <returns>The supplied <see cref="TracerProviderBuilder"/> for chaining.</returns>
         public static TracerProviderBuilder AddProcessor<T>(this TracerProviderBuilder tracerProviderBuilder)
             where T : BaseProcessor<Activity>
         {
-            return ConfigureBuilder(
-                tracerProviderBuilder,
-                (sp, provider) => provider.AddProcessor(sp.GetRequiredService<T>()));
+            return tracerProviderBuilder
+                .ConfigureServices(services => services.TryAddSingleton<BaseProcessor<Activity>, T>());
         }
 
         /// <summary>