I am having a problem with converting a string to property.

Example of what do I have:

Expression<Func<string, bool>> expression = (a => a == "Hello");

What do I want:

Expression<Func<Entity, bool>> expression = (a => a.Name == "Hello");

However, I need the mentioned method to do the following as well:

Expression<Func<string, bool>> expression = (a => "Hello" == a);

=>

Expression<Func<Entity, bool>> expression = (a => "Hello" == a.Name);

What I think might work, but I am unable to do it:

• I think that a visitor could be a way how to properly do this. However, I am not sure how to properly handle it.

CodePudding user response：

You need to create new lambda expression where parameter has type Entity and body is a body of original expression where string parameter replaced by accessor to Name property. ExpressionVisitor is a right tool to do such replacement

    // parameter for new lambda
    var p = Expression.Parameter(typeof(Entity), "p");
    // Expression for p.Name
    var exprProp = Expression.Property(p, nameof(Entity.Name));
    // replace parameter in original expression 
    var visitor = new ReplaceParamVisitor(expression.Parameters[0], exprProp);
    var exprNewBody = visitor.Visit(expression.Body);
    // create new lambda
    var lambda = Expression.Lambda<Func<Entity, bool>>(exprNewBody, p);

Visitor:

public class ReplaceParamVisitor: ExpressionVisitor
{
    private readonly ParameterExpression param;
    private readonly Expression replacement;

    public ReplaceParamVisitor(ParameterExpression param, Expression replacement)
    {
        this.param = param;
        this.replacement = replacement;
    }

    protected override Expression VisitParameter(ParameterExpression node) => node == param ? replacement : node;
}