I have an Expression in the following form:
Expression<Func<T, bool>> predicate = t => t.Value == "SomeValue";
Is it possible to create a 'partially applied' version of this expression:
Expression<Func<bool>> predicate = () => t.Value == "SomeValue";
NB This expression is never actually compiled or invoked, it is merely inspected to generate some SQL.
This could be easily achieved by writing a custom ExpressionVisitor and replacing the parameter with a constant expression that you have captured in a closure:
public class Foo
{
public string Value { get; set; }
}
public class ReplaceVisitor<T> : ExpressionVisitor
{
private readonly T _instance;
public ReplaceVisitor(T instance)
{
_instance = instance;
}
protected override Expression VisitParameter(ParameterExpression node)
{
return Expression.Constant(_instance);
}
}
class Program
{
static void Main()
{
Expression<Func<Foo, bool>> predicate = t => t.Value == "SomeValue";
var foo = new Foo { Value = "SomeValue" };
Expression<Func<bool>> result = Convert(predicate, foo);
Console.WriteLine(result.Compile()());
}
static Expression<Func<bool>> Convert<T>(Expression<Func<T, bool>> expression, T instance)
{
return Expression.Lambda<Func<bool>>(
new ReplaceVisitor<T>(instance).Visit(expression.Body)
);
}
}