??最近框架中的可視化界面設(shè)計(jì)需要使用到表達(dá)式引擎(解析代碼字符串并動(dòng)態(tài)執(zhí)行)，之前舊框架的實(shí)現(xiàn)是將表達(dá)式字符串解析為語(yǔ)法樹(shù)后解釋執(zhí)行該表達(dá)式，本文介紹如何使用Roslyn解析表達(dá)式字符串，并直接轉(zhuǎn)換為L(zhǎng)inq的表達(dá)式后編譯執(zhí)行。

一、語(yǔ)法(Syntax)與語(yǔ)義(Semantic)

??C#的代碼通過(guò)Roslyn解析為相應(yīng)的語(yǔ)法樹(shù)，并且利用語(yǔ)義分析可以獲取語(yǔ)法節(jié)點(diǎn)所對(duì)應(yīng)的符號(hào)及類(lèi)型信息，這樣利用這些信息可以正確的轉(zhuǎn)換為L(zhǎng)inq的表達(dá)式。這里作者就不展開(kāi)了，可以參考Roslyn文檔。

• 語(yǔ)法分析
  https://learn.microsoft.com/en-us/dotnet/csharp/roslyn-sdk/get-started/syntax-analysis
• 語(yǔ)義分析
  https://learn.microsoft.com/en-us/dotnet/csharp/roslyn-sdk/get-started/semantic-analysis
• 語(yǔ)法轉(zhuǎn)換
  https://learn.microsoft.com/en-us/dotnet/csharp/roslyn-sdk/get-started/syntax-transformation

二、實(shí)現(xiàn)表達(dá)式解析器(ExpressionParser)

1. 解析字符串方法

??下面開(kāi)始創(chuàng)建一個(gè)類(lèi)庫(kù)工程，引用包Microsoft.CodeAnalysis.CSharp.Features，然后參照以下代碼創(chuàng)建ExpressionParser類(lèi), 靜態(tài)ParseCode()方法是解析字符串表達(dá)式的入口:

using System.Linq.Expressions;
using Microsoft.CodeAnalysis;
using Microsoft.CodeAnalysis.CSharp;
using Microsoft.CodeAnalysis.CSharp.Syntax;

namespace ExpEngine;

public sealed class ExpressionParser : CSharpSyntaxVisitor<Expression>
{
    private ExpressionParser(SemanticModel semanticModel)
    {
        _semanticModel = semanticModel;
    }

    private readonly SemanticModel _semanticModel;

    /// <summary>
    /// 解析表達(dá)式字符串轉(zhuǎn)換為L(zhǎng)inq的表達(dá)式
    /// </summary>
    public static Expression ParseCode(string code)
    {
        var parseOptions = new CSharpParseOptions().WithLanguageVersion(LanguageVersion.CSharp11);
        var compilationOptions = new CSharpCompilationOptions(OutputKind.DynamicallyLinkedLibrary)
            .WithNullableContextOptions(NullableContextOptions.Enable);

        var tree = CSharpSyntaxTree.ParseText(code, parseOptions);
        var root = tree.GetCompilationUnitRoot();
        var compilation = CSharpCompilation.Create("Expression", options: compilationOptions)
            .AddReferences(MetadataReference.CreateFromFile(typeof(string).Assembly.Location))
            .AddSyntaxTrees(tree);
        var semanticModel = compilation.GetSemanticModel(tree);
        //檢查是否存在語(yǔ)義錯(cuò)誤
        var diagnostics = semanticModel.GetDiagnostics();
        var errors = diagnostics.Count(d => d.Severity == DiagnosticSeverity.Error);
        if (errors > 0)
            throw new Exception("表達(dá)式存在語(yǔ)義錯(cuò)誤");

        var methodDecl = root.DescendantNodes().OfType<MethodDeclarationSyntax>().First();
        if (methodDecl.Body != null && methodDecl.Body.Statements.Count > 1)
            throw new NotImplementedException("Parse block body");

        if (methodDecl.ExpressionBody != null)
            throw new NotImplementedException("Parse expression body");

        var firstStatement = methodDecl.Body!.Statements.FirstOrDefault();
        if (firstStatement is not ReturnStatementSyntax returnNode)
            throw new Exception("表達(dá)式方法不是單行返回語(yǔ)句");

        var parser = new ExpressionParser(semanticModel);
        return parser.Visit(returnNode.Expression)!;
    }
}

2. 解析運(yùn)行時(shí)類(lèi)型的方法

??因?yàn)檗D(zhuǎn)換過(guò)程中需要將Roslyn解析出來(lái)的類(lèi)型信息轉(zhuǎn)換為對(duì)應(yīng)的C#運(yùn)行時(shí)的類(lèi)型，所以需要實(shí)現(xiàn)類(lèi)型轉(zhuǎn)換的方法：

    private readonly Dictionary<string, Type> _knownTypes = new()
    {
        { "bool", typeof(bool) },
        { "byte", typeof(byte) },
        { "sbyte", typeof(sbyte) },
        { "short", typeof(short) },
        { "ushort", typeof(ushort) },
        { "int", typeof(int) },
        { "uint", typeof(uint) },
        { "long", typeof(long) },
        { "ulong", typeof(ulong) },
        { "float", typeof(float) },
        { "double", typeof(double) },
        { "char", typeof(char) },
        { "string", typeof(string) },
        { "object", typeof(object) },
    };

    /// <summary>
    /// 根據(jù)類(lèi)型字符串獲取運(yùn)行時(shí)類(lèi)型
    /// </summary>
    private Type ResolveType(string typeName)
    {
        if (_knownTypes.TryGetValue(typeName, out var sysType))
            return sysType;

        //通過(guò)反射獲取類(lèi)型
        var type = Type.GetType(typeName);
        if (type == null)
            throw new Exception($"Can't find type: {typeName} ");

        return type;
    }

3. 解析各類(lèi)語(yǔ)法節(jié)點(diǎn)轉(zhuǎn)換為對(duì)應(yīng)的Linq表達(dá)式

??這里舉一個(gè)簡(jiǎn)單的LiteralExpression轉(zhuǎn)換的例子，其他請(qǐng)參考源碼。需要注意的是Linq的表達(dá)式嚴(yán)格匹配類(lèi)型簽名，比如方法調(diào)用object.Equals(object a, object b), 如果參數(shù)a是int類(lèi)型，需要使用Expression.Convert(int, typeof(object))轉(zhuǎn)換為相應(yīng)的類(lèi)型。

    private Type? GetConvertedType(SyntaxNode node)
    {
        var typeInfo = _semanticModel.GetTypeInfo(node);
        Type? convertedType = null;
        if (!SymbolEqualityComparer.Default.Equals(typeInfo.Type, typeInfo.ConvertedType))
            convertedType = ResolveType((INamedTypeSymbol)typeInfo.ConvertedType!);

        return convertedType;
    }

    public override Expression? VisitLiteralExpression(LiteralExpressionSyntax node)
    {
        var convertedType = GetConvertedType(node);
        var res = Expression.Constant(node.Token.Value);
        return convertedType == null ? res : Expression.Convert(res, convertedType);
    }

三、測(cè)試解析與執(zhí)行表達(dá)式

??現(xiàn)在可以創(chuàng)建一個(gè)單元測(cè)試項(xiàng)目驗(yàn)證一下解析字符串表達(dá)式并執(zhí)行了，當(dāng)然實(shí)際應(yīng)用過(guò)程中應(yīng)緩存解析并編譯的表達(dá)式委托:

namespace UnitTests;

using static ExpEngine.ExpressionParser;

public class Tests
{
    [Test]
    public void StaticPropertyTest() => Assert.True(Run<object>("DateTime.Today") is DateTime);

    [Test]
    public void InstancePropertyTest() => Run<int>("DateTime.Today.Year");

    [Test]
    public void MethodCallTest1() => Run<DateTime>("DateTime.Today.AddDays(1 + 1)");

    [Test]
    public void MethodCallTest2() => Run<DateTime>("DateTime.Today.AddDays(DateTime.Today.Year)");

    [Test]
    public void MethodCallTest3() => Run<DateTime>("DateTime.Today.AddDays(int.Parse(\"1\"))");

    [Test]
    public void MethodCallTest4() => Assert.True(Run<bool>("Equals(new DateTime(1977,3,1), new DateTime(1977,3,1))"));

    [Test]
    public void PrefixUnaryTest() => Run<DateTime>("DateTime.Today.AddDays(-1)");

    [Test]
    public void NewTest() => Assert.True(Run<DateTime>("new DateTime(1977,3,16)") == new DateTime(1977, 3, 16));

    [Test]
    public void BinaryTest1() => Assert.True(Run<float>("3 + 2.6f") == 3 + 2.6f);

    [Test]
    public void BinaryTest2() => Assert.True(Run<bool>("3 >= 2.6f"));
}

四、 一些限制與TODO

??Linq的表達(dá)式本身存在一些限制，請(qǐng)參考文檔:
https://learn.microsoft.com/en-us/dotnet/csharp/advanced-topics/expression-trees/

??另上述代碼僅示例，比如表達(dá)式輸入?yún)?shù)等未實(shí)現(xiàn)，小伙伴們可以繼續(xù)自行完善。

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