最大长度验证控件：MaxLengthValidator

如果数据库字段为varchar或char类型，ASP.NET控件在可输入汉字的情况下，MaxLength属性不能保证在保存到数据库时不发生截断错误，因此写了一个最大长度验证控件，还可用于多行文本框。

/// <summary>
    /// TextBox的最大长度验证器，如果是varchar，char字段类型，需要此验证
    /// </summary>
    [Description("对MaxLength进行验证")]
    [ToolboxData("<{0}:MaxLengthValidator runat=server></{0}:MaxLengthValidator>")]
    public class MaxLengthValidator : BaseValidator
    {
        /// <summary>
        /// 获取或设置是否为非nvarchar,nchar字段验证
        /// </summary>
        [DefaultValue(true)]
        public bool ValidateForNonNationalField
        {
            get
            {
                var data = this.ViewState["ValidateForNonNationalField"];
                if (data == null)
                {
                    return true;
                }
                else
                {
                    return (bool)data;
                }
            }
            set
            {
                this.ViewState["ValidateForNonNationalField"] = value;
            }
        }

        /// <summary>
        /// 构造函数
        /// </summary>
        public MaxLengthValidator()
        {
            this.ValidateForNonNationalField = true;
        }

        protected override bool EvaluateIsValid()
        {
            var targetControlName = this.ControlToValidate;
            if (targetControlName == null)
            {
                //没有目标验证控件，验证通过
                return true;
            }

            var targetControl = this.NamingContainer.FindControl(targetControlName) as TextBox;
            if (targetControl == null)
            {
                //目标控件不是文本框或不存在，验证通过
                return true;
            }

            var maxLength = targetControl.MaxLength;
            var targetValue = this.GetControlValidationValue(targetControlName);
            var targetLength = 0;
            if (string.IsNullOrEmpty(targetValue))
            {
                targetLength = 0;
            }
            else
            {
                if (this.ValidateForNonNationalField)
                {
                    targetLength = this.GetLengthForNonNationnal(targetValue);
                }
                else
                {
                    targetLength = targetValue.Length;
                }
            }

            return targetLength <= maxLength;
        }

        /// <summary>
        /// 获得保存为Varchar时，text的长度
        /// </summary>
        /// <param name="text"></param>
        /// <returns></returns>
        public virtual int GetLengthForNonNationnal(string text)
        {
            if (string.IsNullOrEmpty(text))
            {
                return 0;
            }

            var length = 0;
            for (var i = 0; i < text.ToCharArray().Length; i++)
            {
                var charCode = (int)text[i];
                
                length++;
                if (charCode > 255)
                {
                    length++;
                }
            }
            return length;
        }

        protected override void AddAttributesToRender(HtmlTextWriter writer)
        {            
            base.AddAttributesToRender(writer);

            if (this.RenderUplevel)
            {
                writer.AddAttribute("TargetControlID",
                    this.GetControlRenderID(this.ControlToValidate));
                writer.AddAttribute("ValidateForNonNationalField",
                    this.ValidateForNonNationalField ? "true" : "false");
                writer.AddAttribute("evaluationfunction",
                             "MaxLengthValidator_Validate");

                var targetControl = this.NamingContainer.FindControl(this.ControlToValidate) as TextBox;
                if (targetControl != null)
                {
                    var maxLength = targetControl.MaxLength;
                    writer.AddAttribute("maxlength",
                        maxLength.ToString());
                }
            }
        }

        protected override void OnPreRender(EventArgs e)
        {
            var script = @"
function MaxLengthValidator_GetNonNationalLength(text) {
    if (!text){
        return 0;
    }
    var length = 0;
    for (var i = 0; i < text.length; i++) {
        var charCode = text.charCodeAt(i);
        length++;
        if (charCode > 255) {
            length++;
        }
    }
    return length;
}
function MaxLengthValidator_Validate(val) {
    if (!val) {
        return true;
    }    
    var targetID = val.TargetControlID;
    if (!targetID) {
        return true;
    }
    var nonNational = val.ValidateForNonNationalField;
    if (!nonNational) {
        return true;
    } 
    var target = document.getElementById(targetID);
    if (!target) {
        return true;
    }
    var maxLength = val.maxlength;
    if (!maxLength || isNaN(maxLength)) {
        return true;
    }
    var targetValue = target.value;
    if (!targetValue) {
        return true;
    }
    var realLength = targetValue.length;
    if (nonNational == 'true')
    {
        realLength = MaxLengthValidator_GetNonNationalLength(targetValue);
    }
    return realLength <= parseInt(maxLength);
}";
            ScriptManager.RegisterClientScriptBlock(this,
                typeof(MaxLengthValidator),
                "MaxLengthValidator_Validate",
                script,
                true);
            
            base.OnPreRender(e);
        }
    }