c# 如何在 returns 字符串的表达式中计算 compareTo(..)

Question

我和这里有类似的情况：How to declare a Linq Expression variable in order to have it processed as a dbParameter

但在我的例子中，不想与 'equal' 运算符进行比较，我需要将一个字符串与另一个字符串进行比较。我怎样才能做到这一点？

我试过这样的事情：

    var comparsionString = "aaa";
    ParameterExpression param = Expression.Parameter(typeof(ItemSearch), "s");
    Expression prop = Expression.Property(param, "Id");
    Expression<Func<string>> idLambda = () => comparsionString;
    Expression searchExpr = Expression.GreaterThanOrEqual(prop, idLambda.Body);

    Expression<Func<ItemSearch, bool>> myLambda =
        Expression.Lambda<Func<ItemSearch, bool>>(searchExpr, param);

但不幸的是，字符串不支持 GreaterThanOrEqual。所以我需要 string.CompareTo(..) 方法，它至少被 SQL 支持到 Entities.

说：

Expression searchExpr = Expression.IsTrue(*prop*.CompareTo(*idLambda*) >= 0);

如何以编译器可以理解的方式编写？

感谢任何帮助。谢谢！

Answer 1

如果您调查

的编译器翻译

Expression<Func<string, bool>> f = s => s.CompareTo("aaaa") >= 0;

你会发现你需要使用 GreaterThanOrEqual(Call("s", "CompareTo", "aaaa")) 所以创建的代码是：

var comparsionString = "aaa";
ParameterExpression param = Expression.Parameter(typeof(ItemSearch), "s");
Expression prop = Expression.Property(param, "Id");
Expression<Func<string>> idLambda = () => comparsionString;
var CallMethod = typeof(string).GetMethod("CompareTo", new[] { typeof(string) });
Expression callExpr = Expression.Call(prop, CallMethod, idLambda.Body);
Expression searchExpr = Expression.GreaterThanOrEqual(callExpr, Expression.Constant(0));

Expression<Func<ItemSearch, bool>> myLambda =
    Expression.Lambda<Func<ItemSearch, bool>>(searchExpr, param);

c# 如何在 returns 字符串的表达式中计算 compareTo(..)

c# How to evaluate compareTo(..) in Expression which returns a string

c#

linq

lambda

linq-expressions