用ReplacingExpressionVisitor替换自定义ExpressionVisitor

Question

我让下面的代码正常工作。

但是，我不想像现在这样使用定制的ParameterReplacer类，我更希望我可以去掉这个类，而使用ReplacingExpressionVisitor。但是我似乎找不到正确的语法，在那里我可以获得现有的参数，以便可以用parameter替换它。

GetDateExpression

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

ParameterReplacer

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

实际上最好在合并前替换参数：

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，如果您重用来自GetNullableDateExpression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;