用 ReplacingExpressionVisitor 替换自定义 ExpressionVisitor

Question

我可以使用下面的代码。

但是，与其像我一样使用自定义的 ParameterReplacer class，我更愿意删除此 class 并改用 ReplacingExpressionVisitor。但我似乎找不到正确的语法来获取现有参数，以便我可以用 parameter.

替换它

获取日期表达式

protected readonly Expression<Func<T, DateTime>> GetDateExpression;

参数替换器

internal class ParameterReplacer : ExpressionVisitor
{
    private readonly ParameterExpression Parameter;

    internal ParameterReplacer(ParameterExpression parameter)
    {
        Parameter = parameter;
    }

    protected override Expression VisitParameter(ParameterExpression node)
    {
        return base.VisitParameter(Parameter);
    }
}

主要代码

Expression expression;
Expression startExpression, endExpression;

// Build sub expressions
var parameter = Expression.Parameter(typeof(T));
startExpression = startDate.HasValue ?
    Expression.GreaterThanOrEqual(GetNullableDateExpression.Body, Expression.Constant(startDate, typeof(DateTime?))) :
    null;
endExpression = endDate.HasValue ?
    Expression.LessThanOrEqual(GetNullableDateExpression.Body, Expression.Constant(endDate, typeof(DateTime?))) :
    null;

// Build main expression
if (startExpression != null && endExpression != null)
    expression = Expression.AndAlso(startExpression, endExpression);
else if (startExpression != null)
    expression = startExpression;
else
    expression = endExpression;

// TODO: Change to use ReplacingExpressionVisitor instead

// Use our real parameter
expression = new ParameterReplacer(parameter)
    .Visit(expression);

// Modify query
return query.Where(Expression.Lambda<Func<T, bool>>(expression, parameter));

Answer 1

实际上在组合之前替换参数更好：

Expression expression;
Expression startExpression, endExpression;

// Build sub expressions
var parameter = Expression.Parameter(typeof(T));

var nullableExpressionBody = ReplacingExpressionVisitor.Replace(GetNullableDateExpression.Body, GetNullableDateExpression.Parameters[0], parameter);

startExpression = startDate.HasValue ?
    Expression.GreaterThanOrEqual(nullableExpressionBody, Expression.Constant(startDate, typeof(DateTime?))) :
    null;
endExpression = endDate.HasValue ?
    Expression.LessThanOrEqual(nullableExpressionBody, Expression.Constant(endDate, typeof(DateTime?))) :
    null;

// Build main expression
if (startExpression != null && endExpression != null)
    expression = Expression.AndAlso(startExpression, endExpression);
else if (startExpression != null)
    expression = startExpression;
else
    expression = endExpression;

// Modify query
return query.Where(Expression.Lambda<Func<T, bool>>(expression, parameter));

此外，如果您重复使用 GetNullableDateExpression lambda 中的参数，则不需要替换。

// Build sub expressions
var parameter = GetNullableDateExpression.Parameters[0];

startExpression = startDate.HasValue ?
    Expression.GreaterThanOrEqual(GetNullableDateExpression.Body, Expression.Constant(startDate, typeof(DateTime?))) :
    null;
endExpression = endDate.HasValue ?
    Expression.LessThanOrEqual(GetNullableDateExpression.Body, Expression.Constant(endDate, typeof(DateTime?))) :
    null;

用 ReplacingExpressionVisitor 替换自定义 ExpressionVisitor

Replace custom ExpressionVisitor with ReplacingExpressionVisitor

c#

linq

expression-trees

.net-5

获取日期表达式

参数替换器

主要代码