从实体框架表中清除列值

Question

I need to clean the value of a column in C# using linq.

Here is an example:

var p_name = "michaelscott";
var result = db.tablename.where(x => x.name
        .Replace(" ",'')
        .Replace("$","") 
        == p_name).FirstOrDefault();

This example only uses 2 items (spaces and dollar sign). if I wanted to add more things to clean, the linq query would be enormous, so I tried writing a method but it throws an error that this wasn't allowed. I then tried writing an extension class that did all the replaces with in it but it errors out with the same error as using a method.

Is there a way to do this without having to use a .Replace for each character I want to remove?

Answer 1

Run an ad-hoc query within EF to easily acomplish what you're trying to do.

var cmd = "UPDATE dbo.TableName SET Name = REPLACE(REPLACE(t.Column, ' ', ''), '$', '')";
    cmd += "FROM Table t";
    cmd += "WHERE t.Name = value;";

context.Database.ExecuteSqlCommand(cmd);

Answer 2

Replaced with (I think) a better answer.

You can write a method to generate the proper Expression to represent the cleaned-up value using an ExpressionVisitor . The easiest way to use it is to write a new version of Where so that type inference will work for you.

public static class IQueryableExt {
    // body only for LINQ to Objects use
    public static string RemoveAll(this string src, string removeChars) => removeChars.Aggregate(src, (ans,ch) => ans.Replace(ch.ToString(), ""));

    private static Expression CleanUp(this Expression dbFn, string charsToRemove) {
        var toCharE = Expression.Constant(String.Empty);
        var replaceMI = typeof(string).GetMethod("Replace", new[] { typeof(string), typeof(string) });

        var methodBody = dbFn;
        foreach (var ch in charsToRemove)
            methodBody = Expression.Call(methodBody, replaceMI, Expression.Constant(ch.ToString()), toCharE);

        return methodBody;
    }

    /// <summary>
    /// ExpressionVisitor to expand x.RemoveAll("x..z") to x.Replace("x","")...Replace("z","") in
    /// an Expression.
    /// </summary>
    private class RemoveAllVisitor : ExpressionVisitor {
        public override Expression Visit(Expression node) {
            if (node?.NodeType == ExpressionType.Call) {
                var callnode = (MethodCallExpression)node;
                if (callnode.Method.Name == "RemoveAll" && callnode.Method.DeclaringType == typeof(IQueryableExt))
                    if (callnode.Arguments[1] is ConstantExpression ce && ce.Type == typeof(string))
                        return callnode.Arguments[0].CleanUp(ce.Value.ToString());
            }

            return base.Visit(node);
        }
    }

    private static T ExpandRemoveAll<T>(this T orig) where T : Expression => (T)new RemoveAllVisitor().Visit(orig);

    public static IQueryable<T> WhereRemoveAll<T>(this IQueryable<T> src, Expression<Func<T, bool>> predFn) => src.Where(predFn.ExpandRemoveAll());
}

With the extension code in place, using it is relatively easy:

var p_name = "michaelscott";
var result = db.tablename
                 .WhereRemoveAll(x => x.name.RemoveAll("$ ") == p_name)
                 .FirstOrDefault();

A more general approach might use something like LINQKit's approach to modify the query and automatically expand all embedded methods with some attribute - I added some code using this approach on CodeReview here .

Answer 3

you can use a Regex expression to catch all the characters you'd like to replace

 var result = db.tablename(x => 
                Regex.Replace(x, @"[^\w]", "")
          == p_name).FirstOrDefault();

The above expression will remove all chars that are not a letter or number