Replacing the parameter name in the Body of an Expression
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);
}