Replacing the parameter name in the Body of an Exp

问题:

I'm trying to dynamically build up expressions based on a Specification object.

I've created an ExpressionHelper class that has a private Expression like so:

private Expression<Func<T, bool>> expression;

public ExpressionHelper()
{
    expression = (Expression<Func<T, bool>>)(a => true);
}

And then some easy methods as follows:

public void And(Expression<Func<T,bool>> exp);

I'm struggling with the body of the And method. I basically want to rip the body out of exp, replace all the parameters with those in expression and then append it to the end of the expression body as and AndAlso.

I've done this:

var newBody = Expression.And(expression.Body,exp.Body);

expression = expression.Update(newBody, expression.Parameters);

But that ends up with my expression looking like this:

{ a => e.IsActive && e.IsManaged }

Is there a simpler way to do this? Or how can I rip out those e's and replace them with a's?

回答1:

The simplest approach here is Expression.Invoke, for example:

public static Expression<Func<T, bool>> AndAlso<T>(
    Expression<Func<T, bool>> x, Expression<Func<T, bool>> y)
{
    return Expression.Lambda<Func<T, bool>>(
        Expression.AndAlso(x.Body, Expression.Invoke(y, x.Parameters)),
        x.Parameters);
}

This works fine for LINQ-to-Objects and LINQ-to-SQL, but isn't supported by EF. For EF you'll need to use a visitor to rewrite the tree, sadly.

Using the code from: Combining two lambda expressions in c#

public static Expression<Func<T, bool>> AndAlso<T>(
    Expression<Func<T, bool>> x, Expression<Func<T, bool>> y)
{
    var newY = new ExpressionRewriter().Subst(y.Parameters[0], x.Parameters[0]).Inline().Apply(y.Body);

    return Expression.Lambda<Func<T, bool>>(
        Expression.AndAlso(x.Body, newY),
        x.Parameters);
}

Or in .NET 4.0, using ExpressionVisitor:

class ParameterVisitor : ExpressionVisitor
{
    private readonly ReadOnlyCollection<ParameterExpression> from, to;
    public ParameterVisitor(
        ReadOnlyCollection<ParameterExpression> from,
        ReadOnlyCollection<ParameterExpression> to)
    {
        if(from == null) throw new ArgumentNullException("from");
        if(to == null) throw new ArgumentNullException("to");
        if(from.Count != to.Count) throw new InvalidOperationException(
             "Parameter lengths must match");
        this.from = from;
        this.to = to;
    }
    protected override Expression VisitParameter(ParameterExpression node)
    {
        for (int i = 0; i < from.Count; i++)
        {
            if (node == from[i]) return to[i];
        }
        return node;
    }
}
public static Expression<Func<T, bool>> AndAlso<T>(
      Expression<Func<T, bool>> x, Expression<Func<T, bool>> y)
{
    var newY = new ParameterVisitor(y.Parameters, x.Parameters)
              .VisitAndConvert(y.Body, "AndAlso");
    return Expression.Lambda<Func<T, bool>>(
        Expression.AndAlso(x.Body, newY),
        x.Parameters);
}