LINQ Expression Tree - The parameter 'x' was not bound in the specified LINQ to Entities query expression

Question

I am trying to build an expression tree dynamically to fetch data from a database.

The following codes are used for this.

Expression<Func<Client, bool>> expression = x => true;
foreach (var item in searchParams)
{
    var operatorType = ExpressionType.Equal;
    string propertyName = null;
    object value = null;
    string keyValue = item.Value;

    if (item.Key == Constants.SearchParameterNames.Id)
    {
        int val = 0;
        if (int.TryParse(keyValue, out val))
            value = val;
        propertyName = "ClientID";
    }
    else if (item.Key == Constants.SearchParameterNames.Lastupdate)
    {
        DateTime dateTime;
        if (DateTime.TryParse(keyValue, out dateTime))
            value = dateTime;
        propertyName = "LastChange";
    }

    if (!string.IsNullOrWhiteSpace(propertyName) && value != null)
    {

            var exp = GetBinaryOperation<Client>(propertyName, operatorType, value);
            var exp1 = Expression.And(expression.Body, exp);
            expression = Expression.Lambda<Func<Client, bool>>(exp1, expression.Parameters);
    }

}
var client = _clientRepository.FindBy(expression).ToList();

when _clientRepository.FindBy(expression).ToList() is executed I am getting an exception of

The parameter 'x' was not bound in the specified LINQ to Entities query expression.

The method used to create expression:

public BinaryExpression GetBinaryOperation<T>(string propertyName, ExpressionType type, object value)
{

    var parameterExpression = Expression.Parameter(typeof(T), "x");
    var memberExpression = Expression.Property(parameterExpression, propertyName); 
    var propertyType = GetMemberType(memberExpression);
    var rhs = Expression.Constant(value);
    var binaryExpression = Expression.MakeBinary(type, memberExpression, rhs);

    return binaryExpression; 
}

Answer 1

When building such an expression you have to preserve the top-level parameter expression instance. When you create a new parameter expression in the GetBinaryOperation function, that will be a different instance (hence the not bound term), regardless of the fact that its name is the same "x".

Instead of creating a new parameter instance, you should pass the original LambdaExpression's "x" parameter to the GetBinaryOperation function using for example expression.Parameters[0].

All in all, you have to use the same parameter expression instance throughout the entire expression tree in this case.