avoid duplicate evaluation in ResolveConnectionStringReferenceAsync (#7714)

Co-authored-by: Adam Ratzman <[email protected]>

Author: adamint

src/Aspire.Hosting/ApplicationModel/ConnectionStringReference.cs

@@ -27,9 +27,11 @@ public class ConnectionStringReference(IResourceWithConnectionString resource, b
 
         if (string.IsNullOrEmpty(value) && !Optional)
         {
-            throw new DistributedApplicationException($"The connection string for the resource '{Resource.Name}' is not available.");
+            ThrowConnectionStringUnavailableException();
         }
 
         return value;
     }
+
+    internal void ThrowConnectionStringUnavailableException() => throw new DistributedApplicationException($"The connection string for the resource '{Resource.Name}' is not available.");
 }

src/Aspire.Hosting/ApplicationModel/ExpressionResolver.cs

@@ -156,10 +156,14 @@ async Task<ResolvedValue> ResolveConnectionStringReferenceAsync(ConnectionString
     {
         // We are substituting our own logic for ConnectionStringReference's GetValueAsync.
         // However, ConnectionStringReference#GetValueAsync will throw if the connection string is not optional but is not present.
-        // To avoid duplicating that logic, we can defer to ConnectionStringReference#GetValueAsync, which will throw if needed.
-        await ((IValueProvider)cs).GetValueAsync(cancellationToken).ConfigureAwait(false);
+        // so we need to do the same here.
+        var value = await ResolveInternalAsync(cs.Resource.ConnectionStringExpression).ConfigureAwait(false);
+        if (string.IsNullOrEmpty(value.Value) && !cs.Optional)
+        {
+            cs.ThrowConnectionStringUnavailableException();
+        }
 
-        return await ResolveInternalAsync(cs.Resource.ConnectionStringExpression).ConfigureAwait(false);
+        return value;
     }
 
     /// <summary>