Fix the string interpolation issue (Azure#48988)

* add a solution for simplicity

* have a try

* fix the issue

* add a new case

* refactor test cases

* just noticed that we have a sln

* update test case data

* refactor

* resolve comments

* fix test cases

Author: ArcturusZhang

sdk/provisioning/Azure.Provisioning/src/Expressions/BicepStringBuilder.cs

@@ -104,6 +104,10 @@ public void AppendFormatted<T>(T t)
             _expressions.Add(b.Compile());
             _isSecure = _isSecure || b.IsSecure;
         }
+        else if (t is BicepExpression exp)
+        {
+            _expressions.Add(exp);
+        }
         else
         {
             string? s = t?.ToString();

sdk/provisioning/Azure.Provisioning/tests/Expressions/InterpolatedStringExpressionTest.cs

@@ -0,0 +1,36 @@
+// Copyright (c) Microsoft Corporation. All rights reserved.
+// Licensed under the MIT License.
+
+using Azure.Provisioning.Expressions;
+using NUnit.Framework;
+
+namespace Azure.Provisioning.Tests.Expressions
+{
+    public class InterpolatedStringExpressionTest
+    {
+        [Test]
+        public void ValidateBicepFunctionInterpolate()
+        {
+            // test provisionable variable
+            AssertExpression(
+                "'Var=${foo}'",
+                BicepFunction.Interpolate(
+                    $"Var={new ProvisioningVariable("foo", typeof(string))}"
+                    )
+                );
+
+            // test index expression in interpolation
+            AssertExpression(
+                "'Endpoint=${foo['bar']}'",
+                BicepFunction.Interpolate(
+                    $"Endpoint={new IndexExpression(
+                        new IdentifierExpression("foo"),
+                        new StringLiteralExpression("bar")
+                    )}")
+                );
+
+            static void AssertExpression(string expected, BicepValue<string> expression)
+                => Assert.AreEqual(expected, expression.ToString());
+        }
+    }
+}