Creating a LINQ Expression where parameter equals object

Question

Given a primitive value age I know how to create an expression like this:

//assuming: age is an int or some other primitive type
employee => empl         


        

Answer 1

In addition to what has been mentioned in previous answers. A more specific solution would go as such:

public static Expression CreateExpression(string propertyName, object valueToCompare)
{
    // get the type of entity
    var entityType = typeof(T);
    // get the type of the value object
    var valueType = valueToCompare.GetType();
    var entityProperty = entityType.GetProperty(propertyName);
    var propertyType = entityProperty.PropertyType;


    // Expression: "entity"
    var parameter = Expression.Parameter(entityType, "entity");

    // check if the property type is a value type
    // only value types work 
    if (propertyType.IsValueType || propertyType.Equals(typeof(string)))
    {
        // Expression: entity.Property == value
        return Expression.Equal(
            Expression.Property(parameter, entityProperty),
            Expression.Constant(valueToCompare)
        );
    }
    // if not, then use the key
    else
    {
        // get the key property
        var keyProperty = propertyType.GetProperties().FirstOrDefault(p => p.GetCustomAttributes(typeof(KeyAttribute), false).Length > 0);

        // Expression: entity.Property.Key == value.Key
        return Expression.Equal(
            Expression.Property(
                Expression.Property(parameter, entityProperty),
                keyProperty
            ),
            Expression.Constant(
                keyProperty.GetValue(valueToCompare),
                keyProperty.PropertyType
            )
        );
    }
}

IMPORTANT POINTS :

1. Make sure to check for nulls
2. Make sure propertyType and valueType are compatible (either they are the same type or are convertible)
3. Several assumptions are made here (e.g. that you do assign a KeyAttribute)
4. This code is not tested, so it is not exactly copy/paste ready.

Hope that helps.