CSharp之Expression

简介

• 表达式树，当前只是我抄袭别人的代码，基本的逻辑和应该如何扩展我还是不清楚，所以不做过多简介，等我之后弄清楚我会继续更新。这篇主要是为了有个参考的代码（写的简洁比较逻辑较清晰），同时也是为了可以随时查看，上传到博客记录。

基本代码

public class MyExpression : ExpressionVisitor
{
    private Stack<string> values = new Stack<string>();
    public MyExpression()
    {

    }
    public string Condition()
    {
        var condition = string.Concat(values.ToArray());
        values.Clear();
        return condition;
    }

    public string ToSqlOpearator(ExpressionType expressionType)
    {
        var op = default(string);
        switch (expressionType)
        {
            case ExpressionType.Add:
                break;
            case ExpressionType.AddChecked:
                break;
            case ExpressionType.And:
                break;
            case ExpressionType.AndAlso:
                op = "AND";
                break;
            case ExpressionType.ArrayLength:
                break;
            case ExpressionType.ArrayIndex:
                break;
            case ExpressionType.Call:
                break;
            case ExpressionType.Coalesce:
                break;
            case ExpressionType.Conditional:
                break;
            case ExpressionType.Constant:
                break;
            case ExpressionType.Convert:
                break;
            case ExpressionType.ConvertChecked:
                break;
            case ExpressionType.Divide:
                break;
            case ExpressionType.Equal:
                op = "=";
                break;
            case ExpressionType.ExclusiveOr:
                break;
            case ExpressionType.GreaterThan:
                op = ">";
                break;
            default:
                break;

        }

        return op;
    }

    protected override Expression VisitBinary(BinaryExpression node)
    {
        Console.WriteLine($"node {node}");
        if (node == null)
        {
            return null;
        }
        values.Push(")");
        Visit(node.Right);
        values.Push($"{ToSqlOpearator(node.NodeType)}");

        Visit(node.Left);
        values.Push("(");
        return node;
    }

    protected override Expression VisitMember(MemberExpression node)
    {
        if (node == null)
        {
            return null;
        }

        values.Push(node.Member.Name);
        return node;
    }

    protected override Expression VisitConstant(ConstantExpression node)
    {
        if (node == null)
        {
            return null;
        }
        values.Push(node.Value.ToString());
        return node;
    }
    protected override Expression VisitMethodCall(MethodCallExpression node)
    {
        if (node == null)
        {
            return null;
        }
        Visit(node.Object);
        Visit(node.Arguments[0]);
        var right = values.Pop();
        var left = values.Pop();

        switch (node.Method.Name)
        {
            case "StartsWith":
                values.Push($"({left} LIKE '{right}%')");
                break;
            default:
                break;
        }
        return node;

    }

}

//调用方法如下
 Expression<Func<User, bool>> exp = (x) => x.Name.StartsWith("Peng") && x.Age > 18 && x.Age == 10;
 var myExpression = new MyExpression();
 myExpression.Visit(exp);
 var sql = myExpression.Condition();

待续