Add server variable substitution logic.

src/Microsoft.OpenApi/Extensions/OpenApiServerExtensions.cs

@@ -0,0 +1,57 @@
+using System;
+using System.Collections.Generic;
+using Microsoft.OpenApi.Models;
+using Microsoft.OpenApi.Properties;
+
+namespace Microsoft.OpenApi.Extensions;
+
+/// <summary>
+/// Extension methods for <see cref="OpenApiServer"/> serialization.
+/// </summary>
+public static class OpenApiServerExtensions
+{
+    /// <summary>
+    /// Replaces URL variables in a server's URL
+    /// </summary>
+    /// <param name="server">The OpenAPI server object</param>
+    /// <param name="values">The server variable values that will be used to replace the default values.</param>
+    /// <returns>A URL with the provided variables substituted.</returns>
+    /// <exception cref="ArgumentException">
+    /// Thrown when:
+    ///   1. A substitution has no valid value in both the supplied dictionary and the default
+    ///   2. A substitution's value is not available in the enum provided
+    /// </exception>
+    public static string ReplaceServerUrlVariables(this OpenApiServer server, IDictionary<string, string> values = null)
+    {
+        var parsedUrl = server.Url;
+        foreach (var variable in server.Variables)
+        {
+            // Try to get the value from the provided values
+            if (values is not { } v || !v.TryGetValue(variable.Key, out var value) || string.IsNullOrEmpty(value))
+            {
+                // Fall back to the default value
+                value = variable.Value.Default;
+            }
+
+            // Validate value
+            if (string.IsNullOrEmpty(value))
+            {
+                // According to the spec, the variable's default value is required.
+                // This code path should be hit when a value isn't provided & a default value isn't available
+                throw new ArgumentException(
+                    string.Format(SRResource.ParseServerUrlDefaultValueNotAvailable, variable.Key), nameof(server));
+            }
+
+            // If an enum is provided, the array should not be empty & the value should exist in the enum
+            if (variable.Value.Enum is {} e && (e.Count == 0 || !e.Contains(value)))
+            {
+                throw new ArgumentException(
+                    string.Format(SRResource.ParseServerUrlValueNotValid, value, variable.Key), nameof(values));
+            }
+
+            parsedUrl = parsedUrl.Replace($"{{{variable.Key}}}", value);
+        }
+
+        return parsedUrl;
+    }
+}

src/Microsoft.OpenApi/Models/OpenApiDocument.cs

@@ -8,6 +8,7 @@
 using System.Security.Cryptography;
 using System.Text;
 using Microsoft.OpenApi.Exceptions;
+using Microsoft.OpenApi.Extensions;
 using Microsoft.OpenApi.Interfaces;
 using Microsoft.OpenApi.Services;
 using Microsoft.OpenApi.Writers;
@@ -283,14 +284,7 @@
 
         private static string ParseServerUrl(OpenApiServer server)
         {
-            var parsedUrl = server.Url;
-
-            var variables = server.Variables;
-            foreach (var variable in variables.Where(static x => !string.IsNullOrEmpty(x.Value.Default)))
-            {
-                parsedUrl = parsedUrl.Replace($"{{{variable.Key}}}", variable.Value.Default);
-            }
-            return parsedUrl;
+            return server.ReplaceServerUrlVariables(new Dictionary<string, string>(0));
         }
 
         private static void WriteHostInfoV2(IOpenApiWriter writer, IList<OpenApiServer> servers)
@@ -441,7 +435,7 @@
                 return null;
             }
 
             // Todo: Verify if we need to check to see if this external reference is actually targeted at this document.
             if (useExternal)
             {
                 if (this.Workspace == null)

src/Microsoft.OpenApi/Models/OpenApiServerVariable.cs

@@ -26,7 +26,10 @@ public class OpenApiServerVariable : IOpenApiSerializable, IOpenApiExtensible
         /// <summary>
         /// An enumeration of string values to be used if the substitution options are from a limited set.
         /// </summary>
-        public List<string> Enum { get; set; } = new();
+        /// <remarks>
+        /// If the server variable in the OpenAPI document has no <code>enum</code> member, this property will be null.
+        /// </remarks>
+        public List<string> Enum { get; set; }
 
         /// <summary>
         /// This object MAY be extended with Specification Extensions.

src/Microsoft.OpenApi/Properties/SRResource.resx

@@ -225,4 +225,10 @@
   <data name="WorkspaceRequredForExternalReferenceResolution" xml:space="preserve">
     <value>OpenAPI document must be added to an OpenApiWorkspace to be able to resolve external references.</value>
   </data>
+  <data name="ParseServerUrlDefaultValueNotAvailable" xml:space="preserve">
+      <value>Invalid server variable '{0}'. A value was not provided and no default value was provided.</value>
+  </data>
+  <data name="ParseServerUrlValueNotValid" xml:space="preserve">
+      <value>Value '{0}' is not a valid value for variable '{1}'. If an enum is provided, it should not be empty and the value provided should exist in the enum</value>
+  </data>
 </root>

src/Microsoft.OpenApi/Validations/Rules/OpenApiServerRules.cs

@@ -26,9 +26,32 @@ public static class OpenApiServerRules
                         context.CreateError(nameof(ServerRequiredFields),
                             String.Format(SRResource.Validation_FieldIsRequired, "url", "server"));
                     }
+
+                    context.Exit();
+                    context.Enter("variables");
+                    foreach (var variable in server.Variables)
+                    {
+                        context.Enter(variable.Key);
+                        ValidateServerVariableRequiredFields(context, variable.Key, variable.Value);
+                        context.Exit();
+                    }
                     context.Exit();
                 });
 
         // add more rules
+
+        /// <summary>
+        /// Validate required fields in server variable
+        /// </summary>
+        private static void ValidateServerVariableRequiredFields(IValidationContext context, string key, OpenApiServerVariable item)
+        {
+            context.Enter("default");
+            if (string.IsNullOrEmpty(item.Default))
+            {
+                context.CreateError("ServerVariableMustHaveDefaultValue",
+                    String.Format(SRResource.Validation_FieldIsRequired, "default", key));
+            }
+            context.Exit();
+        }
     }
 }

test/Microsoft.OpenApi.Readers.Tests/V3Tests/OpenApiDocumentTests.cs

@@ -1367,5 +1367,70 @@ public void ParseDocumetWithWrongReferenceTypeShouldReturnADiagnosticError()
             diagnostic.Errors.Should().BeEquivalentTo(new List<OpenApiError> {
                 new( new OpenApiException("Invalid Reference Type 'Schema'.")) });
         }
+
+        [Fact]
+        public void ParseBasicDocumentWithServerVariableShouldSucceed()
+        {
+            var openApiDoc = new OpenApiStringReader().Read("""
+                                                            openapi : 3.0.0
+                                                            info:
+                                                                title: The API
+                                                                version: 0.9.1
+                                                            servers:
+                                                              - url: http://www.example.org/api/{version}
+                                                                description: The http endpoint
+                                                                variables:
+                                                                  version:
+                                                                    default: v2
+                                                                    enum: [v1, v2]
+                                                            paths: {}
+                                                            """, out var diagnostic);
+
+            diagnostic.Should().BeEquivalentTo(
+                new OpenApiDiagnostic { SpecificationVersion = OpenApiSpecVersion.OpenApi3_0 });
+
+            openApiDoc.Should().BeEquivalentTo(
+                new OpenApiDocument
+                {
+                    Info = new()
+                    {
+                        Title = "The API",
+                        Version = "0.9.1",
+                    },
+                    Servers =
+                    {
+                        new OpenApiServer
+                        {
+                            Url = "http://www.example.org/api/{version}",
+                            Description = "The http endpoint",
+                            Variables = new Dictionary<string, OpenApiServerVariable>
+                            {
+                                {"version", new OpenApiServerVariable {Default = "v2", Enum = ["v1", "v2"]}}
+                            }
+                        }
+                    },
+                    Paths = new()
+                });
+        }
+
+        [Fact]
+        public void ParseBasicDocumentWithServerVariableAndNoDefaultShouldFail()
+        {
+            var openApiDoc = new OpenApiStringReader().Read("""
+                                                            openapi : 3.0.0
+                                                            info:
+                                                                title: The API
+                                                                version: 0.9.1
+                                                            servers:
+                                                              - url: http://www.example.org/api/{version}
+                                                                description: The http endpoint
+                                                                variables:
+                                                                  version:
+                                                                    enum: [v1, v2]
+                                                            paths: {}
+                                                            """, out var diagnostic);
+
+            diagnostic.Errors.Should().NotBeEmpty();
+        }
     }
 }

test/Microsoft.OpenApi.Tests/Extensions/OpenApiServerExtensionsTests.cs

@@ -0,0 +1,104 @@
+using System;
+using System.Collections.Generic;
+using FluentAssertions;
+using Microsoft.OpenApi.Extensions;
+using Microsoft.OpenApi.Models;
+using Xunit;
+
+namespace Microsoft.OpenApi.Tests.Extensions;
+
+public class OpenApiServerExtensionsTests
+{
+    [Fact]
+    public void ShouldSubstituteServerVariableWithProvidedValues()
+    {
+        var variable = new OpenApiServer
+        {
+            Url = "http://example.com/api/{version}",
+            Description = string.Empty,
+            Variables = new Dictionary<string, OpenApiServerVariable>
+            {
+                { "version", new OpenApiServerVariable { Default = "v1", Enum = ["v1", "v2"]} }
+            }
+        };
+
+        var url = variable.ReplaceServerUrlVariables(new Dictionary<string, string> {{"version", "v2"}});
+
+        url.Should().Be("http://example.com/api/v2");
+    }
+
+    [Fact]
+    public void ShouldSubstituteServerVariableWithDefaultValues()
+    {
+        var variable = new OpenApiServer
+        {
+            Url = "http://example.com/api/{version}",
+            Description = string.Empty,
+            Variables = new Dictionary<string, OpenApiServerVariable>
+            {
+                { "version", new OpenApiServerVariable { Default = "v1", Enum = ["v1", "v2"]} }
+            }
+        };
+
+        var url = variable.ReplaceServerUrlVariables(new Dictionary<string, string>(0));
+
+        url.Should().Be("http://example.com/api/v1");
+    }
+
+    [Fact]
+    public void ShouldFailIfNoValueIsAvailable()
+    {
+        var variable = new OpenApiServer
+        {
+            Url = "http://example.com/api/{version}",
+            Description = string.Empty,
+            Variables = new Dictionary<string, OpenApiServerVariable>
+            {
+                { "version", new OpenApiServerVariable { Enum = ["v1", "v2"]} }
+            }
+        };
+
+        Assert.Throws<ArgumentException>(() =>
+        {
+            variable.ReplaceServerUrlVariables(new Dictionary<string, string>(0));
+        });
+    }
+
+    [Fact]
+    public void ShouldFailIfProvidedValueIsNotInEnum()
+    {
+        var variable = new OpenApiServer
+        {
+            Url = "http://example.com/api/{version}",
+            Description = string.Empty,
+            Variables = new Dictionary<string, OpenApiServerVariable>
+            {
+                { "version", new OpenApiServerVariable { Enum = ["v1", "v2"]} }
+            }
+        };
+
+        Assert.Throws<ArgumentException>(() =>
+        {
+            variable.ReplaceServerUrlVariables(new Dictionary<string, string> {{"version", "v3"}});
+        });
+    }
+
+    [Fact]
+    public void ShouldFailIfEnumIsEmpty()
+    {
+        var variable = new OpenApiServer
+        {
+            Url = "http://example.com/api/{version}",
+            Description = string.Empty,
+            Variables = new Dictionary<string, OpenApiServerVariable>
+            {
+                { "version", new OpenApiServerVariable { Enum = []} }
+            }
+        };
+
+        Assert.Throws<ArgumentException>(() =>
+        {
+            variable.ReplaceServerUrlVariables(new Dictionary<string, string> {{"version", "v1"}});
+        });
+    }
+}

test/Microsoft.OpenApi.Tests/PublicApi/PublicApi.approved.txt

@@ -285,6 +285,10 @@ namespace Microsoft.OpenApi.Extensions
         public static void SerializeAsYaml<T>(this T element, System.IO.Stream stream, Microsoft.OpenApi.OpenApiSpecVersion specVersion)
             where T : Microsoft.OpenApi.Interfaces.IOpenApiSerializable { }
     }
+    public static class OpenApiServerExtensions
+    {
+        public static string ReplaceServerUrlVariables(this Microsoft.OpenApi.Models.OpenApiServer server, System.Collections.Generic.IDictionary<string, string> values = null) { }
+    }
     public static class OpenApiTypeMapper
     {
         public static System.Type MapOpenApiPrimitiveTypeToSimpleType(this Microsoft.OpenApi.Models.OpenApiSchema schema) { }